In order to evaluate age-related changes in ovarian and adrenal steroid production, in vitro steroid production by adrenal glands and ovaries from young (3-4 mo) and middle-aged (10-11 mo) cycling rats was compared to serum steroid and gonadotropin levels on each morning of the estrous cycle. Basal LH levels were not different between young and mid-aged cycling rats except on estrus, when elevated estrogen (E) levels were correlated with depressed LH in the mid-aged rats. Basal FSH levels were generally elevated in mid-aged cycling and mid-aged constant estrus (CE) rats, but the FSH rise on estrus morning was not seen in the mid-aged rats. Serum progesterone levels were not changed with age or reproductive state, although in vitro ovarian progesterone secretion was decreased in mid-aged CE rats. Adrenal progesterone secretion increased significantly with age. Serum total testosterone was similar in young and mid-aged cycling and mid-aged CE rats, despite a highly significant increase in in vitro testosterone secretion by the CE ovary. Serum estradiol (E2) levels were significantly elevated on proestrus and estrus in the mid-aged rats. Although estrone (E1) levels appeared higher in the mid-aged than in the young cycling rats, the differences were not significant. Mid-aged CE rats had significantly elevated serum levels of both E1 and E2. In vitro ovarian estrone production was depressed in mid-aged cycling rats. Adrenal total estrogen production was similar in young and mid-aged animals. These results demonstrate that serum gonadotropin and steroid levels are altered in aging female rats prior to the loss of reproductive cycles. Changes in serum steroid levels are probably due to changes in circulating LH and FSH levels or the ovarian response to these gonadotropins, but changes in vitro basal steroid production suggest that intrinsic ovarian function may also change with advancing age. As rats enter a CE state, alterations in basal ovarian and adrenal steroid production are seen and may be partially responsible for maintenance of the acyclic state.