The effects of physiological and pharmacological variations of oestrogens on prolactin and thyrotrophin (TSH) secretion have been studied during the menstrual cycle and under oral contraceptive treatment. Ten women were tested for prolactin and TSH responses to 200 mug TRH in the early follicular (days 4-6), periovulatory (days 14-15) and luteal phases (days 22-26) of the same menstrual cycle. Circulating plasma prolactin levels did not significantly vary in the three phases, but TSH basal levels were lower in the luteal than in the follicular and periovulatory periods. The prolactin response to TRH was significantly enhanced in the periovulatory phase, while the TSH response was slightly decreased. Seven women on sequential contraceptives exhibited increased basal and TRH-induced prolactin secretion during the oestrogen treatment, with an unaltered TSH secretion throughout therapy. Treatment with combined contraceptives did not alter either basal or TRH-induced prolactin secretion in eight women, but basal TSH secretion and its response to TRH were both reduced. These data show that oestrogens may produce different regulatory effects on prolactin and TSH secretion, particularly in the pituitary sensitivity to TRH stimulation. Physiological variations of oestrogen secretion such as those observed during the menstrual cycle can likewise modify prolactin levels. These results could provide some support for a regulatory role for prolactin in the menstrual cycle.