Scientists Colin Blakemore and Gill Langley square up for tonight's Controversial Thesis debate at the National Portrait Gallery. They will be confronting questions such as is there a good enough reason to use animals in medical research if it relieves suffering in humans? Is such research reliable and is it right to award ourselves a superior moral status to other creatures?

About 2.7 million animals a year are used in the UK in biomedical research for experiments ultimately aimed at benefiting humans and animals. Any decent person would agree that it isn't right to kill animals without a very good reason. So, is the reason good enough?

That question is the crux of the argument about animal research. First, a little arithmetic: 2.7 million is fewer than half the number of animals used in the early 1970s, but it is still a very large figure. But do the sums and you will find that it translates to about three animals for each person in the country over their entire lifetime.

Since about 85 per cent of the animals used in research are rodents, that approximates to two mice and half a rat used on your behalf during the whole of your life. (Monkeys, the target of such passionate argument, account for just 0.1 per cent of animals used.) Compare that with the vastly greater number of rats and mice killed as vermin, or with the 600 chickens eaten by the average Briton during a lifetime, or the regiments of pigs whose skins provide our shoes, or the legions of unwanted pets that are destroyed each year, or hunted foxes, or beasts of burden, or the circus performers. It's always struck me as odd that, of all the ways in which human beings use or kill animals, the one that has the objective of improving health, rather than promoting pleasure or vanity is the one most vociferously criticised by animal-rights campaigners.

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But is animal research justified and worth while? On average, Britons have gained about three months of extra life for every year of the past century.

Much of this increase in life expectancy has come from improvements in hygiene and nutrition. But a lot has been due to the reduction in infant mortality through vaccines and other treatments. And there has been a real rise in life expectancy for people of every age.

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Think of the lives saved by antibiotics, blood transfusion, insulin, transplantation and coronary bypass surgery, by treatments for leukaemia and breast cancer, and by drugs for ulcers, high blood pressure and asthma.

Think of the better anaesthetics, painkillers, antidepressants and treatments for Aids. And think, too, about all the treatments that vets can offer animals. These are the extraordinary benefits of medical research.

Not all of that research has involved animals. Indeed, the law states that animals can be used only if there is no alternative. The Medical Research Council spends far more on research on isolated molecules, cells and tissues, computer modelling and human clinical trials than it spends on research on laboratory animals. But Petri dishes don't get heart attacks; computers don't have strokes. Animal research is crucial to bridge the gap of knowledge between the test tube and the bedside.

It's easy to produce exaggerated, stomach-churning misrepresentations of animal experiments, or to claim that they just don't work, or that there are alternatives for all of them. But if we want cures for the terrible conditions that destroy the lives of humans and animals, research - some of it on animals - must continue.

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If you want to hear the real case for this research, listen to people dying from cancer or cystic fibrosis, or to the families of those destroyed by stroke or Alzheimer's. While animal research is still essential, though, it is important to make every effort to reduce animal use where possible and improve their treatment - and this must underpin our approach to the future of medical research.

The views of the vast majority of scientists and doctors on this issue are crystal clear. The Royal Society, for instance, issued a booklet earlier this year that states: "Humans have benefited immensely from scientific research involving animals, with virtually every medical achievement in the past century reliant on the use of animals in some way." Even more important, the latest opinion polls show that more than 90 per cent of the British people accept the necessity of animal research, as long as there is no alternative, as long as suffering is kept to the absolute minimum and as long as the work is done for a good cause. Can you think of any other current public issue on which 90 per cent of the public share the same view?

The fact is that medical research on animals is no longer a controversial issue. The British public, including all the research scientists I know, would love to see an end to animal research. But for the moment, they accept the argument: two mice and half a rat isn't too high a price to pay.

Colin Blakemore is chief executive of the Medical Research Council.

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A few tickets are still available for the debate tonight (Thursday). Call 0207 306 0055, ext 216.

Upcoming debates: Thursday May 20, 7pm. John Gray will propose "The ethics of George W. Bush: Good, bad or irrelevant?". Peter Singer will oppose.

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Thursday June 17, 7pm. Donald Kuspit will propose "Embalm a modern artist and give beauty a chance". Norman Rosenthal will oppose.