Newborn rat pups were either handled for 3 min/day for the first 24 days of life or else they were not disturbed during this interval. After weaning at 25 days, females were raised either in groups or individually. Rupturing of the vaginal membrane and onset of estrus occurred significantly earlier for females which were handled in infancy and group-reared after weaning. Males were sacrificed at 35, 41 or 47 days; the testes, prostates and seminal vesicles were excised and weighed. The seminal vesicles and prostates of handled rats were found to be significantly heavier than those of unhandled controls. No differences were found for testes weight.