The secretory function of both human ovaries was studied simultaneously by comparing the concentrations of several steroids in the effluent blood from the two ovaries with their concentrations in the peripheral blood. As expected, progesterone and estradiol emerged as important secretory products. In both the preovulatory and postovulatory phases of the menstrual cycle, significant amounts of 17α-hydroxyprogesterone and androstenedione were found. In two cases dehydroepiandrosterone was present in high concentration in ovarian vein blood. 20α-hydroxy-4-pregnene-3-one, but not 20β-hydroxy-4-pregnene-3-one, was consistently secreted by the corpus luteum. Small amounts of estrone and testosterone may be secreted by the ovary. In two ases with bilateral ovulation, there was a similarity between the steroid output of the two corpora lutea. In one subject studied following clomiphene treatment, anatomical as well as biochemical vidence of a hyperstimulated state was demonstrated. None of the steroids studied were secreted by the ovaries of a postmenopausal woman with the possible exception of androstenedione. While the bulk of the hormones was secreted by the ovary bearing the ripe follicle or the corpus luteum, the data indicate that the contralateral ovary actively secretes androgenic steroid hormones, especially androstenedione.