Studies on the effects of progesterone on rat adrenal cortex were performed on two age groups of intact or ovariectomized female rats. Ovariectomy was performed on animals weighing 150-170 g and autopsies were carried out 6 weeks and 8.5 months after surgery (4- and 11-month-old animals). 12 days before autopsy part of the animals were treated with progesterone at a daily dose of 5 mg per rat, the others received a single injection of depot-estradiol or jointly with progesterone and estradiol. Intact female rats of a corresponding age were treated with progesterone or vehicle only. In younger intact female rats progesterone lowers the relative adrenal weight and corticosterone output by whole adrenal homogenates. In older rats despite the lowering in absolute and relative adrenal weights there were no changes in corticosterone output. In ovariectomized rats of both age groups progesterone had no marked effects on studied parameters while estradiol resulted in an increase in adrenal weights and corticosterone output. Part of the changes evoked by estradiol were prevented by concomitant progesterone administration. The above described changes were accompanied by morphologic alterations. Performed studies show that progesterone effects on the rat adrenal cortex are modified by the ovaries and depend on the age of animals.