: The possible influence of exogenous melatonin upon the reproductive axis of middle-aged (15-19-month-old) female rats showing irregular estrous cycle length was analyzed. In vitro pituitary and median eminence (ME) luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin release and responsiveness to luteinizing hormone releasing hormone (LHRH) were investigated in control (n=12) and melatonin (150 &mgr;g/100 g BW) treated (n=16) rats. In vitro ovarian steroidogenic activity and its response to hCG were also evaluated. Basal secretion, first incubation after one hour (I(1)), second incubation after 2 hours (I(2)) and after two hours plus LHRH (10(-7)) (I(2+LHRH)) or hCG (20 UI) (I(2+hCG)) were studied. Melatonin administration to middle-aged rats showed a different effect upon in vitro LH release than upon FSH since only lower FSH secretion rates were found. In vivo melatonin administration reduced in vitro LH release after LHRH stimulation and this was impaired to pituitary LH content, indicating that in vivo melatonin administration alters the mechanism of LH release but does not affect the LH synthesis. Neither melatonin nor LHRH affected hemipituitary in vitro prolactin release. In vivo melatonin administration again showed a different effect upon LH release than upon FSH from ME, showing decreasing FSH secretion rates. Similarly, melatonin did not affect prolactin release or ME content. Melatonin reduced estradiol release from hCG stimulated ovaries of middle-aged rats. We concluded that melatonin may have a physiological role in middle-aged rats since decreased in vitro basal FSH release and blunted LH and estradiol increase after stimuli.