Plasma levels of progesterone have been measured, using a competition binding technique, in the systemic circulation of normal and castrated female and male rats of the Sprague Dawley strain. Plasma progesterone exhibits a large fluctuation in normal females, with a peak during the day of proestrus. In females, progesterone does not disappear completely from the systemic circulation after castration, even if its levels are much lower than those of normally cycling animals. Progesterone is detectable in the systemic circulation of normal males, and its levels increase significantly after orchidectomy. The ip administration of 25 μg/100 g body wt of dexamethasone (a potent ACTH-blocking agent) to female animals castrated IS days previously induces, in 4 hr, a significant decrease of plasma levels of progesterone. Plasma levels of corticosterone (measured with a fluorometric procedure) are also significantly decreased after dexamethasone. Exogenous ACTH (administered iv 15 min before sacrifice) to dexamethasone-treated castrated females brings back to normal plasma levels of both progesterone and corticosterone. Ovine prolactin, ovine GH and a preparation of human LH administered in the same experimental conditions enhance plasma levels of progesterone, but do not modify plasma levels of corticosterone. Ovine LH is ineffective on both progesterone and corticosterone, while ovine FSH decreases plasma levels of progesterone but leaves plasma corticosterone unmodified. It is concluded that: a) the adrenal gland of both male and female rats secretes measurable amounts of progesterone; b) the secretion of adrenal progesterone is regulated by ACTH, prolactin, GH and possibly LH; c) prolactin seems to play a major role in such a regulation, because of the magnitude of the effects it induces; and d) the hypersecretion of adrenal progesterone which occurs during the day of proestrus (and which seems to be important for facilitating the release of ovulatory amounts of LH and for entertaining female mating behavior) may be due to the combined action of ACTH and prolactin. (Endocrinology93: 1178, 1973)