A population of captive-reared wild Norway rats and two stocks of domestic rats were compared for various parameters of sexual maturation and fecundity. Males did not differ in age at the first appearance of spermatozoa in the testes but wild males were significantly older than Long--Evans and Sprague--Dawley domestic males at first copulation with a hormone-induced oestrous female. Wild females were significantly older than domestic females at the time of vaginal opening, first oestrus and first conception. Stocks did not differ in length of the first oestrous cycle. Wild females produced significantly smaller litters than their domestic counterparts. Domestic rats were significantly heavier than wild rats at the various stages of sexual maturation. The results support the hypothesis that domestication of the rat has been accompanied by genetic and/or environmental changes that accelerate sexual development and improve breeding success under laboratory conditions.