In female rat age-related reproductive decline is accompanied by progressive impairment of the neuroendocrine mechanisms that regulate LH secretion. The biosynthetic activity of the pineal gland is markedly depressed and the nocturnal secretion of melatonin decreases significantly. The aim of the present study was to evaluate whether the nocturnal administration of melatonin via the drinking water (0.4 µg/ml) throughout the course of aging from 14 to 24 months of age could (1) influence the age-related changes that occur in basal serum levels of LH and in the LH response to GnRH or to naloxone stimulation at 16, 18 and 20 months of age, and (2) delay the onset of the postreproductive constant estrous-anovulatory state as evaluated by the daily recording of vaginal smears and by occurrence of polyfollicular ovaries at 24 months of age. Our results demonstrate that melatonin replacement delays the increase in LH serum levels and the decrease in LH response to GnRH that occur in 18-month-old control animals. Furthermore, they show that melatonin treatment prevents the loss of LH response to naloxone manifested in control rats between 16 and 20 months of age. Melatonin also appears to prevent the progressive increase in the monthly occurrence of estrus phases as well as to decrease the number of rats with polyfollicular ovaries at 24 months of age in comparison to control animals. These results suggest that the age-related decrease in circulating melatonin during the night may contribute to the reproductive decline of aging, and that this effect may involve the central opioid system