The effect of tamoxifen (TAM) on the serum levels of sexual hormones and on the sex hormone-binding globulin (SHBG) was investigated in 30 postmenopausal patients with advanced breast cancer. To study the 'prolactin reserve capacity' of the pituitary gland, thyrotrophin-releasing hormone (TRH) and sulpiride-induced prolactin release were measured prior to TAM treatment, then in the 2nd and 8th week of the therapy. The TRH (400 micrograms i.v.)-induced prolactin secretion was significantly suppressed by TAM after an 8-week treatment, but only in responding cases. Maximal prolactin stimulation occurred at the 15th min after TRH injection, being equal to 5,600 +/- 800 mlU/l in cancer patients, and decreasing to 2,400 +/- 150 mlU/l after 8 weeks. TAM did not suppress the sulpiride-inducable prolactin release either in responders or in nonresponders.