Photoperiod is an important environmental cue timing puberty in the domestic female sheep, a seasonal breeder. Because the effects of photoperiod on reproductive function in the adult sheep are mediated by the pineal melatonin rhythm, the present study determined whether the pineal through its secretion of melatonin is involved in the pubertal process. Neonatal denervation of the pineal by removal of the superior cervical ganglia (SGX) abolished the nocturnal melatonin rhythm and delayed the onset of reproductive cycles beyond 1 yr of age in female lambs reared under natural conditions. Puberty was also delayed in SGX lambs reared under an artificial photoperiod sequence of long days (experienced at 17-22 weeks of age) followed by short days. By comparison, intact lambs under this same photoperiod treatment began cycles at the normal age. Another group of SGX lambs was treated each night with melatonin; the infusion replicated the duration of the nighttime rise in serum melatonin occurring in intact lambs exposed to 5 weeks of long days followed by short days (9-h or 15-h infusions, respectively). Melatonin treatment of these SGX lambs restored puberty to the normal age. These findings suggest that the pineal melatonin rhythm is an essential component of the photoperiodic mechanism timing puberty in the female sheep.