Ovarian hyperstimulation syndrome occurred after induction of ovulation with menotropins (follicle-stimulating hormone and luteinizing hormone) and implantation of an intrauterine pregnancy. Serial determinations of aldosterone, deoxycorticosterone, 17β-estradiol, progesterone, human chorionic gonadotropin, urinary and plasma electrolytes, and fluid balance were obtained. Plasma renin activity, aldosterone, deoxycorticosterone, and antidiuretic hormone rose markedly. Hydration for four days improved urinary output but also accelerated sodium and fluid retention. Subsequent restriction of salt and water stabilized the patient. Spontaneous abortion was followed by prompt diuresis without a change in therapy. Regression analysis of the authors' data, the clinical observations, and other data in the literature suggest that the ovarian hyperstimulation syndrome is produced by excessive secretion of an unknown hormone that regulates peritoneal fluid during the normal menstrual cycle, and that elevations of plasma renin, aldosterone, and antidiuretic hormone are secondary.