OBJECTIVE To evaluate the effects of a 6 month administration of raloxifene hydrochloride, a selective estrogen receptor modulator which was recently approved for the prevention of osteoporosis, on serum gonadotropin and prolactin (PRL) levels and on TRH-stimulated PRL responsiveness in postmenopausal women who have not undergone estrogen replacement therapy. METHODS Sixteen healthy postmenopausal women were divided into two groups on the basis of their bone status, evaluated by dual energy X-ray absorptiometry at the lumbar level. Eight women (chronological age 52.4+/-4.1 (s.d.) years, menopausal age 42.4+/-3.9 years), in whom T-score L2-L4 was less than -2.5 s.d., were treated with raloxifene (60 mg p.o.) administered daily for 6 months (group 1), while the other eight women (chronological age 52.6+/-2.5 years, menopausal age 42.1+/-3.6 years), in whom the T-score L2-L4 ranged between -1 and -2.5 s.d., were used as a control group (group 2). Serum PRL, FSH, LH and 17beta-estradiol (E2) levels were evaluated at baseline and after 3 and 6 months of treatment. In all subjects, PRL responsiveness to TRH (200 microg i.v.) administration was evaluated at baseline and at the end of the study. RESULTS At baseline, mean PRL, LH and FSH levels were not significantly different in the two groups (PRL 133.6+/-21.7 vs 136.7+/-28.1 mIU/l (NS), LH 25.1+/-6.8 vs 24.4+/-6.7 mIU/ml (NS), FSH 74.4+/-25.0 vs 71.1+/-24.1 mIU/ml (NS), in group 1 and group 2 respectively). No significant variations in serum FSH and LH values, in either group, or in serum PRL levels in group 2, were observed at the 3 and 6 month examinations. On the contrary, serum PRL values decreased significantly in group 1 after 3 months (100.1+/-47.7 mIU/l, P<0.05) and 6 months (81.5+/-30.2 mIU/l, P<0.001). At baseline, no significant differences were observed in the TRH-stimulated serum PRL peak between the groups (1015.4+/-30.5 vs 1030.2+/-25.7 mIU/l in group 1 and in group 2 respectively), while it decreased significantly at the 6 month examination in group 1 (770.5+/-47.4 mIU/l, P<0.001) and it was significantly lower than in group 2 (1068.1+/-301.8 mIU/l, P=0.02). Serum E2 was not detected at baseline and at each examination, in all patients. CONCLUSIONS The decrease of PRL values induced by long-term raloxifene administration in postmenopausal women could be explained by a direct antiestrogenic effect of raloxifene on lactotrope cells or by the recently suggested increase of opiatergic tone on the hypothalamic-pituitary region.