In her defence of animal experimentation, Theresa Frayn points out that 99 per cent of the animals experimented on are rats and mice (Oxford Mail, August 2). In his book Vivisection Unveiled: An Expos of the Medical Futility of Animal Experimentation (1997), Dr Tony Page investigates why rodents are the species most commonly used in experimental research, and allows the animal researchers to provide some form of explanation.

In the laboratory textbook The Rat in Laboratory Investigation, the two editors, Dr E Farris and Dr J Griffith, inform us: "The albino rat has come to be the most widely-used laboratory animal.

"This extensive use is due to a number of factors such as low cost, small space requirement, tractability (rats have a docile and submissive nature), short time span of generations, large litters . . . rats are less expensive than large animals to buy and maintain, their living quarters require less space and consequently more animals may be used. The animals themselves are easier to handle."

So, in other words, rats are widely used in animal research, not because they are the species closest to humans in terms of their anatomy and physiology (how could anybody argue otherwise?), but simply because they are small and have low maintenance costs, they have a docile nature, they are cheap to buy, they breed quickly, and they are easy to handle.

These are the criteria by which vivisectors base their research.

Animal research is founded on cost and convenience, not on its scientific relevance to humans.

And the public wonder why animal-tested drugs keep on being withdrawn from the market after being found to cause serious, often fatal adverse effects in the human patient.

This is not science - this is scientific fraud.

Today, mice have overtaken rats as the most widely used species, presumably because they are even cheaper to use.

Jim Crawford Burton Place Oxford