Abstract
The variability of the response to drugs of immature rats bred in natural litters was compared with that of rats cross-fostered into all-male or all-female groups of 9, 12 and 14. The variability of ovarian response to gonadotrophin was significantly greater in naturally-bred animals than in those from cross-fostered litters. There was a suggestion of a similar effect in the variability of barbiturate sleeping time. Variability of seminal vesicle weight, used as an index of response to gonadotrophin in males, did not differ significantly between the cross-fostered and natural litters. Unexpectedly, cross-fostered rats gained more weight in the 4 days after weaning than naturally-bred rats.