The aim of the present study was to compare the effect of haloperidol on the secretion of luteinizing hormone (LH) in male and female immature rats. At 12 days of age, haloperidol significantly released LH in females but not in males. The same results were obtained using sulpiride. The LH-releasing effect of haloperidol diminished with age; in 28-day-old rats it could not be evidenced. The influence of early organization of brain structures controlling LH secretion on this effect was then studied. Neonatally androgenized females failed to respond at 12 days of age and in neonatally castrated males the effect of haloperidol on LH followed the same pattern as in normal females. We conclude that the dopaminergic system plays an important inhibitory role in the control of LH in infantile females and not in males, and that this sexual difference depends on early differentiation of the brain.