In the photoinhibited castrated male Syrian hamster, removal of the pineal gland or transfer to long photoperiods was followed by a rapid increase in the serum concentration of FSH. Levels were significantly above those of controls within 10 days. Central passive immunization of pineal-intact photoinhibited castrated animals against melatonin had a stimulatory effect on serum FSH levels, comparable with that observed following pinealectomy or transfer to short days. The effects of pinealectomy were blocked by programmed s.c. infusions of melatonin in a time-dependent manner. Serum FSH levels remained low in animals receiving 100 ng melatonin delivered over 10 h but the same mass of melatonin delivered over 4 h had no effect on the response to pinealectomy. Lesions of the anterior hypothalamus had no effect on the pinealectomy-induced increase of serum FSH in animals receiving saline infusions. However, in lesioned animals, programmed infusions of melatonin were no longer able to suppress the rise in FSH following pinealectomy. These results demonstrated that pineal melatonin is the mediator of central photoperiodic control of FSH secretion. The duration of the melatonin signal determines its effectiveness and an intact anterior hypothalamus is necessary for the signal to be read and/or the appropriate neuroendocrine response expressed.