The purpose of this study was to determine how oophorectomy and different hormone replacement therapy (HRT) regimens using low doses of medroxyprogesterone acetate (MPA, 2.5 mg/day) influence the pituitary-gonadal axis function. Ninety (90) women, who had had regular menses prior to surgery, completed a 1-year follow-up period. Patients were assigned to 5 groups. The first (n = 16) received 0.625 mg/day conjugated equine oestrogens (CEE) cyclically, the second (n = 20) 50 micrograms day transdermal oestradiol (E2) cyclically and the third (n = 15) 0.625 mg/day CEE continuously. These 3 groups also received 2.5 mg MPA sequentially for the last 12 days of HRT administration. The fourth group (n = 20) received 0.625 mg/day CEE and 2.5 mg/day of MPA continuously, while the fifth (n = 19) constituted a control group. After oophorectomy all patients showed increases in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels, and decreases in those of E2, oestrone (E1), prolactin (PRL), sex-hormone-binding globulin (SHBG), androstenedione (delta A4) and testosterone (T). No changes were detected in dehydroepiandrosterone sulphate (DHEA-S) levels. After HRT, decreases in FSH, LH and PRL levels and increases in those of E2, E1 and SHBG were observed, but no changes were seen in T, delta A4 or DHEA-S plasma levels. As the differences that were found cannot be attributed to the presence of ovaries, it is reasonable to assume that they were perhaps due to the treatment. All these changes, with the exception of a decrease in PRL levels, are therefore to be expected after HRT.