Rats and mice used for cancer tests in the US may soon have a diet regimen. Studies show that modern lab rats are about 25 per cent heavier than their ancestors were 20 years ago, which has caused a rise in the natural incidence of mammary and pituitary tumours, interfering with experimental results (Science, Vol 264, No 5163).

A way out of this peculiar problem is reducing the rodents' calorie intake by 10 per cent. Toxicologists feel, however, that a better method would be to replace obese rodents by animals bred for a slower growth rate and reproduce late in life.