The origin of plasma sex hormones in postmenopausal women was studied by determining plasma levels under basal conditions, after ACTH stimulation, and after dexamethasone suppression, as well as after hCG stimulation. Values obtained in postmenopausal women were compared with values observed during the follicular phase of the cycle in young women on the one hand, and with values in ovariectomized women of postmenopausal age on the other hand. All sex steroid levels studied with the exception of estrone, were significantly lower in postmenopausal women than in young women during the early follicular phase of the cycle. In ovariectomized women only androgen levels (testosterone, androstenedione, dihydrotestosterone, and to a lesser extent dehydroepiandrosterone,) were lower than in normal postmenopausal women, estrogen, 17 hydroxyprogesterone, and progesterone levels being similar. ACTH increased all plasma steroid levels except estradiol, whereas after dexamethasone, all sex hormone levels were significantly decreased. hCG stimulation finally caused an increase of borderline statistical significance in testosterone, dehydroepiandrosterone, and 17-hydroxyprogesterone levels. We have concluded from this study that the adrenal cortex is almost the exclusive source of plasma estradiol, estrone, progesterone, and 17OH progesterone and the most important source of plasma dehydroepiandrosterone; that the postmenopausal ovary appears to be responsible for about 50% of plasma testosterone and 30% of androstenedione levels; and that hCG stimulation with 5000 IU daily for 3 days, hardly influences steroid secretion by postmenopausal ovaries.