Plasma hormonal changes were analysed in the patients who conceived single or multiple pregnancy following the HMG-HCG therapy, with or without hyperstimulation syndrome. Serial plasma samples were collected from the cases who were administered HMG (150-225IU/day, for 7-11 days) and HCG (3,000-10,000IU/day, for 2-4 days) consecutively. Plasma levels of FSH, LH, prolactin (PRL), estrone (E1), estradiol (E2), progesterone (P), 17-hydroxyprogesterone (17-P), 20 alpha-dihydroprogesterone (20 alpha-P), 5 alpha-dihydroprogesterone (DHP), testosterone (T), androstenedione (A), dehydroepiandrosterone (DHA) were determined simultaneously using specific radioimmunoassays. Quantitative and qualitative changes in plasma hormonal patterns which were characteristic of the single and twin pregnancies as well as that of hyperstimulation syndrome were elucidated. The case of pregnancy with hyperstimulation showed higher estrogen (especially E2) than that without hyperstimulation at initial HCG injection. In case of twin pregnancies the high levels of E2, progestins and A were stimulated and augmented in accordance with the increase of endogenous HCG secreted from the trophoblast. All the cases showed the decrease of DHA and the increase of A after ovulation, because of the suppression of delta 5 and stimulation of delta 4 pathway. These results indicate that in HMG-HCG therapy the major hormonal production shifts from the ovarian follicles to the atretic follicle and corpus luteum on quantitative or qualitative changes.