Serum concentrations of luteinizing hormone (LH) and testosterone were monitored during the first 9 weeks of life in male pigs. In boars, testosterone was constant during this period, and LH was elevated from birth through day 21 and declined between days 21 and 28 to levels observed in adult boars. Castration on day 3 reduced testosterone concentrations without having an immediate effect on LH, but after day 21, LH was consistently greater in castrated males than in boars. Responsiveness of the early postnatal testes to LH was shown by reduced testosterone concentrations after treatment with LH-antiserum on day 2. Norethindrone had no effect on LH or testosterone when given on day 2 but decreased the concentrations of these hormones in 5-month-old boars. Similarly, estrogen had no effect on LH secretion in 4-day-old boars. In 9-month-old castrated males, estrogen decreased LH concentrations during the first 48 h of daily treatment, but after this, LH increased and returned to pretreatment levels by 88 h. Also, daily estrogen treatment of castrated males induced female lordosis behavior. From these observations we conclude that negative feedback control of LH secretion is inoperative in neonatal male pigs and that exogenous estrogen can induce responses characteristic of females in castrated postpubertal males.