Studies were designed to determine the relative roles of the adrenal and ovary on androgen and progestin secretion in pregnant rats. Steroids were measured in serial plasma samples obtained from pregnant rats that had been ovariectomized, ovariectomized and adrenalectomized, or ovariectomized, adrenalectomized, and hypophysectomized and which had been treated with the steroid analogs dydrogesterone (9 beta,10 alpha-pregna-4, 6-diene-3,20-dione) and diethylstilbestrol [(E)4,4'-(1,2-diethyl-1,2- ethenediyl )bisphenol] to maintain pregnancy. Plasma levels of progesterone and 20 alpha-hydroxy-4-pregnen-3-one in pregnant rats were low on day 10, peaked on day 12 (21 +/- 3 and 8 +/- 1 ng/ml, respectively), and returned to low levels by day 16. Similar patterns of progestin levels were found in pregnant ovariectomized-adrenalectomized and ovariectomized-adrenalectomized-hypophysectomized rats maintained with steroid analogs. However, adrenalectomy caused lower levels of serum progesterone on days 10, 14, 16, and 18, demonstrating that the adrenal contributes progesterone to the pool of steroids. In a second study, plasma testosterone levels were measured from blood obtained from serially bled pregnant ovariectomized-adrenalectomized rats maintained with steroid analogs. The testosterone levels increased from day 10 (128 +/- 26 pg/ml) through day 20 (595 +/- 75 pg/ml) and fell until day 22 (411 +/- 377 pg/ml). These findings suggest that the adrenal cortex contributes to the progesterone pool during pregnancy and indicate that active in vivo feto-placental progesterone and testosterone production occurs throughout pregnancy.