Virgin females of two inbred strains of rats, a Sheffield–Wistar albino (originally derived from Boots Ltd, Nottingham) and a PVG hooded (from the colony of the Department of Zoology, Hull, established from stock of the M.R.C. Laboratories, Carshalton) were used. The strains had been bred in the same premises in Cardiff for 3 years before the experiment and environmental conditions were standardized for all the animals. Diet pellets (Modified Breeding Diet: Pilsbury) and water were freely available. Room temperature was 23–25°C and the artificial lighting schedule was 12 hr light (07.00–19.00 hours)/24 hr. The animals were housed at opposite ends of the room (5·4 × 8·1 × 2·7 m) in mesh-bottomed cages. Females were kept with litter mates until paired; each female remained with the male until a vaginal plug was found or she was obviously pregnant and she was then returned to her litter mates for 14 days before being isolated in a maternity cage. Numbers of young were counted on the day after birth but litters were not handled unless several dead or dying young were seen. The animals were mated at the ages of 8, 12, 16 and 20 weeks during each month of the year. There were 4–8 animals in each group and the time of appearance of vaginal plugs, size of litters and rearing success to weaning at 14 days were recorded.