23-day-old male rats were left intact, rendered blind and anosmic, pinealectomized together with blinding and anosmia, or subcutaneously implanted with graded doses of melatonin in beeswax immediately following surgical blinding and anosmia. 5 weeks later, blind, anosmic animals were found to have significantly depressed anterior pituitary, testicular, and accessory sex organ weights. Both pituitary and plasma prolactin and luteinizing hormone (LH) concentrations were also significantly suppressed. Pinealectomy of blind, anosmic animals completely restored testicular and accessory organ weights. Likewise, pituitary LH and prolactin and plasma LH levels were also restored to intact control levels by pineal removal. Only the highest dose of melatonin (1 mg) restored the testicular and accessory sex organ weights to those of the intact controls. As little as 1 μg melatonin restored plasma and pituitary LH concentrations to the levels of the intact controls. However, none of the dosages of melatonin reversed plasma prolactin concentrations to those of the untreated animals. The decrease in pituitary prolactin induced by blinding and anosmia was reversed by pinealectomy or by the lower doses (1, 50 or 100 μg) of melatonin. These results indicate that melatonin implants can reverse the antigonadotrophic effects of blinding and anosmia in male rats. The minimal dose of melatonin required to restore testicular and accessory sex organ weights in blind, anosmic rats is 1 mg implanted subcutaneously in beeswa