The role of the serotoninergic system in acceleration of the sexual development of domesticated rats (Rattus norvegicus) was assessed. The onset of age-related changes in hypothalamic serotonin during prepubertal period occurred earlier in domesticated than in aggressive male rats. Blockade of the serotoninergic system after p-chlorophenylalanine (PCPA) administration on days 40 and 44 delayed the development of the reproductive system in both aggressive and domesticated males. In 60-day-old rats treated with PCPA, levels of testosterone in plasma and the number of mature spermatozoa in epididymis were decreased compared to controls. At the same time, the administration of PCPA on days 30 and 34 did not modify basal testosterone secretion and other parameters in 60-day-old aggressive rats and produced a decrease similar to PCPA injections on days 40 and 44, although less pronounced, in the weights of testes in domesticated animals. Administration of 5-hydroxytryptophan (5-HTP), a precursor of serotonin synthesis, on days 30, 32, 34, 36 and 38 increased plasma testosterone levels and weights of the sex organs in 60-day-old domesticated males, but did not significantly affect the development of reproductive system in aggressive animals. These data indicate that serotonin stimulates sexual development of males during prepubertal period and this activating effect of serotonin occurs earlier in domesticated than in aggressive males. They also suggest that the acceleration in sexual maturation of domesticated rats could result from changes in the ontogenetic dynamic of hypothalamic serotonin induced by a selection for low aggressiveness towards man.