Awindowless, top-floor room full of rodents. Shelves of clear plastic cages. Tiny, ink dot eyes peering out from every direction. On every cage a card, colour-coded for ease of reference.

Each card containing a humble biography of sorts, such as this one. Species: mouse. Born: 17.4.10. Sex: female. Strain: CDFAH. Date weaned: 6.5.10. Date studied: 1.6.10. Date of completion: 1.7.10.

Everything but a name.

We all wonder, sometimes, what life is all about. Up here, science has an answer.

This mouse, incubating a cancer in her modified human genes – already dead, incinerated and disposed of by the time you read this – is here, they say, so that some of us, perhaps even millions, may one day live a little longer.

That's the meaning of life – engineered to be experimented on in the University of Leicester's medical sciences building.

It's not a concept to feel comfortable with, even if you work here.

"I definitely don't like it," says director of research Professor Mike Barer. "I don't eat meat because I think it causes an unnecessary exploitation of animals.

"This is exploitation but it is necessary exploitation."

They "kill" 101 animals a day here, according to the National Anti-Vivisection Alliance (NAVA). That's 36,773 a year.

Prof Barer does not dispute the stat. He does, though, take issue with NAVA's use of the word "kill".

"They talk of us killing animals as if we've nothing better to do," he sighs. "The implication is that we start out with the attitude of torturing animals. That is completely alien to what we are trying to do."

NAVA is behind Stop the Leicester Animal Lab, a campaign that's got up a considerable head of steam to oppose the university's plan to replace this facility with a £15 million centre for animal research on campus.

A website set up by NAVA contains all sorts of disturbing claims about what allegedly happens behind these closed doors and how the new lab will make things worse.

An activist has been on hunger strike for three days and Buddhists have marched in protest. It's those dire headlines that bring us here today.

It's not easy to get into floor 4 of the medical sciences building.

The university has never let the media in before. They're allowing it, they say, to set the public record straight.

Access was accepted with no preconditions and no promise to push the university's side of the story.

So, here we are, on the threshold, fumbling into surgical scrubs, pulling on polythene overshoes that will stop us contaminating the facility with the outside world.

A security card is swiped, a pin code punched and a pair of heavy orange doors slowly part.

Between 5,000 and 6,000 animals are held here.

First impressions?

A place that's seen better days. Drab. No natural daylight.

Green and orange walls do little to lift the low-wattage gloom from fluorescent ceiling tubes. Vinyl floors. Spotlessly clean. Beginning to crack and curl at the corners.

A smell. Difficult to identify. The hint of urine, perhaps. Faint and indistinct.

Quiet. Just the low hum of the air conditioners. No audible sign of the barking dogs or screeching primates that NAVA claim are secretly experimented on.

That's because no dogs or monkeys are housed here or anywhere else in the university, says Prof Barer.

They work with rodents. Toads are kept for their eggs but they're not killed.

Cooler up here. Considerably more pleasant, on this muggy day, than the stuffy heat of Prof Barer's office downstairs.

This floor is thermostat controlled. The temperature never dips below 19C and doesn't rise above 23C.

"We look after the animals better than the humans here," smiles the professor.

The facility manager, who is also the principal animal care and welfare officer, accompanies us.

He doesn't want his name in the newspaper. Neither does anyone else, apart from Prof Barer.

A disclaimer on the Stop the Leicester Animal Lab website is categorical. It does not condone the harassment of staff.

Nor is it inciting anyone to break the law. Still, you can't be too careful.

Scientists and technicians at other facilities have been targeted before by extremists.

There's considerable nervousness about today, concerns of repercussions.

"This is not of our making," says the in-house veterinary surgeon.

"It is an extremely sad reflection of UK society that this has been allowed to happen."

The facility manager says: "Treat this like a royal visit. If you see a door you want us to open, we'll open it for you."

Room 410 SA/LTH/NSEP seems like as good a place to begin as any.

The acronyms on the door stand for "small animals, long-term holding, non-surgical experimental procedures".

It's a room stocked floor to ceiling with rats. They all look content enough, these white rats with pink eyes and their human genes, which are about to be injected with goodness knows what.

Cages are clean, relatively roomy and well-stocked with plenty of things to gnaw at, burrow through or make nests in.

Clipboards full of completed job sheets show they've been inspected daily and had their cages changed at least weekly.

A big stainless steel cage-washer runs almost constantly in a room down the corridor.

Animals can be monitored every five minutes if the experiment they are undergoing puts them at risk of suffering or stress, says the facility manager.

Home Office inspectors come in every six to eight weeks. They can also make unannounced spot checks.

Every experiment, even on a humble mouse, has to clear a university ethics committee.

We move on. Another room picked at random. This one in darkness. More rats. Part of a behavioural experiment, says Prof Barer.

Every animal in here gets 12 hours of artificial day and 12 hours of artificial night.

Another room. Like something from a premature baby unit, this one. Cages that look more like incubators, full of grey mice.

The gloves used to handle them, sealed to the sides, dangle like rubber udders.

These are new mice, says the facility manager, kept quarantined until it can be certain they pose no risk of infection to the others.

The more you see, the more you realise everything in here is controlled and moderated.

Nothing, not even the wood shavings these rodents use as a bed and toilet, contains a rogue variable.

The shavings are sourced from Finland. They are ground down from white wood aspen trees because red woods contain chemicals that can be harmful to mice.

The chippings are sterilised and irradiated so no bugs or bacteria can influence the results of experiments.

That makes for better science, says Prof Barer.

More importantly, it's good for the rodents. A compromised result might require another experiment.

No-one wants that, he insists, because that will result in more deaths – and that is the last thing anyone wants.

Technicians and scientists do get attached to the animals, says the facility manager.

You might find it hard to believe but some of them even give them names.

Floor 4 of the medical sciences building is a cramped, poorly designed 1960s space that can't be overhauled.

Lifts are in the wrong places, storage rooms are difficult to get to and the animals would benefit from better facilities.

The new £15 million facility will be a quantum leap forward, says Prof Barer.

Yes, it will be bigger, he accepts, but that doesn't mean more animals or experiments.

The university wants to invest in a £2 million MRI scanner.

At the moment, there is no room for it.

This and other equipment, says Prof Barer, should reduce animal experimentation because the technology will enable experiments to yield more results.

If you believe Prof Barer, there's a tar black irony to this.

By resisting the new lab, those well-intentioned Buddhists and others are taking a stand against bringing deaths down.

Opponents, of course, say animal cruelty, like work, inevitably expands to fill the time and space available to it.

We move on to the "Beta Room", a space set up to showcase the possibilities of the new facility.

In here, the mice have more spacious cages. Concertinas of blue pipes hooked up to them provide 50 air changes an hour.

These dry out the bedding and remove gases from their urine and faeces but don't suck out the natural pheromones that make these mice feel and behave like mice.

Anyone with two eyes can see the animals are not neglected.

There aren't any secret rooms full of untold suffering.

No pet shop has this kind of exacting standards. It's unlikely children in care have their health and welfare so closely scrutinised.

That doesn't get us away from a harsh truth. No animal, however well treated, gets out of here alive.

They die in their tens of thousands. You can try to soften it, as they do, by saying animals are "euthanised".

Prof Barer doesn't try to pretend there isn't suffering.

Many of the animals in here are bred to become sick. Cancer is a cruel disease, whether it's overrunning a human being or a mouse.

The crucial thing, says the scientist, is they do everything possible to keep pain and distress to an absolute minimum.

If these experiments could be done in test tubes or on computer models, they would be, he stresses.

Sadly, a dish of cells can't always replicate the complex biology of a living creature.

The benefits of animal research are there for us all to see, say animal test supporters.

Foods which help to prevent cancer have been identified in this Leicester lab, as have new ways to get oxygen into bodies after lung failure. That's the kind of science that is helping desperately premature babies to survive and could yet save thousands in a flu pandemic.

Prof Barer works in the field of TB research.

"Tuberculosis kills two million people every year," he says.

"If I see an opportunity to reduce the suffering caused by that disease through the careful, considered use of animal research, then I will.

"I don't like it, but I think it is justified. As a diabetic, I'm someone whose life expectancy is directly related to discoveries made in animals.

"I completely accept the clear and just argument that says we have no right to do this.

"If you believe that, though, you must not avail yourself of any of the discoveries made through animal research. That means no antibiotics.

"Anything else would be hypocritical."

At the end of the day, it all comes down to a question of belief.

Protestor Chris Williams believes animal experimentation is wrong morally and ethically, and is driven by bad science.

He also believes they are experimenting on dogs and primates in the University of Leicester.

I saw no evidence of it.

It was in a different building, he says.

A categorical denial comes from the university.

The research is being done under the auspices of the Medical Research Council, insists Chris.

That is simply not true, says the university.

I do more digging, finding nothing to support the allegation.

Chris refuses to believe it. Barking has been heard by neighbours. The roof is just the right shape for the ventilation systems needed to house dogs.

"I'm 110 per cent certain they've got dogs in that building and 90 per cent sure they've got primates", he says.

Unless the university is lying to the Home Office and funding the research covertly, he is mistaken.

Chris is a spokesman for the Stop the Leicester Animal Lab protest.

He doesn't have a job. He's been campaigning, pretty much full-time, for the best part of two years.

Chris seems like a nice guy, as considered and thoughtful as Professor Barer.

Like Prof Barer, Chris says he has science on his side.

He's not a scientist but he can quote scientists who don't accept there is a benefit from animal experimentation.

Animals are not humans, he says. Just think of all those drugs that could be life-savers but never get developed because they don't work in mice or primates.

Computer models and stem cell research can deliver more reliable results.

The weight of scientific opinion, though, would appear to disagree.

Vested interests, says Chris. The development of genetically modified animals is a massive money-maker.

"A huge part of the industry relies on grants for animal testing," he says. "Scientists aren't going to speak out and risk losing their jobs."

Chris accuses the university of being economical with the truth.

The same could be said of the Stop the Leicester Animal Website.

None of the horrific photographs it contains – dogs and rats in desperate states – come from the facility at Leicester.

"It wouldn't be a very effective website if we didn't have photos," says Chris.

But the suffering shown on their website doesn't come from Leicester. Perhaps people should be told that.

"All those images were gained from people going undercover," he says.

"I would much rather use photographs of what is going on in the university," says Chris.

"There is no way we could get them."

Chris wants more openness. He would like the Government to order an independent scientific inquiry on the issue of animal experimentation.

If the public heard all the evidence, for and against, they would demand a ban, he believes.

What if they didn't? What if the animal activists and their scientists lost the argument?

They wouldn't lose, says Chris, and he truly believes that.