The aim of the present study was to analyze the effects of exogenous melatonin (MT) upon pituitary and adrenal responsiveness to releasing hormones in different phases of the menstrual cycle. We evaluated the response of FSH and LH to 100 micrograms gonadotropin releasing hormone, of TSH and prolactin (PRL) to 200 micrograms thyrotropin releasing hormone (TRH), and of cortisol to 10 micrograms ACTH 1-17. We studied eight young women with normal ovulatory cycles in the early follicular (days 5-7) and luteal (days 22-24) phases. Stimulation tests were performed at 18.00 in baseline conditions as well as 1 h after oral intake of exogenous MT (2 mg as a gelatine capsule). We did not observe any significant change in FSH, LH, TSH and cortisol responses to their respective releasing hormones in either phase of the cycle. PRL response to TRH was higher after MT in the follicular phase, when evaluated in terms of net increment and integrated area of response (p less than 0.02 versus baseline conditions for both variables). In the luteal phase, we recorded larger interindividual variability and higher responses after MT were observed in five out of eight subjects. These results suggest that MT may play a facilitatory role in the TRH-induced PRL release in women of reproductive age.