The effects of melatonin (25 micrograms s.c. daily in the late afternoon for 10 days) on the ovarian morphometry and the gonadotropin secretion were investigated in food-restricted rats and rats fed ad libitum. In food-restricted rats melatonin produced the significant decrease of the surface area of the ovary and of zona granulosa. Moreover, melatonin treatment of food-restricted rats resulted in significant diminishment of the relative areas of Graafian follicles and zona granulosa estimated in relation to the ovary cross-sectional surface area. On the other hand, melatonin did not produce significant alterations of the morphometric indices of the ovary in rats fed ad libitum. Melatonin administration was found to inhibit the effects of gonadoliberin on gonadotropin release in food-restricted but not in normally-fed rats. These findings suggest that food restriction sensitizes the pituitary-ovarian axis to antigonadotropic melatonin action.