We have recently demonstrated that treatment of pregnant baboons with androstenedione (delta 4 A) at midgestation to increase estrogen production induced a pattern of placental cortisol (F) metabolism which was similar to that at term and resulted in de novo F production by the fetus, presumably by activation of the fetal hypothalamic-pituitary-adrenocortical axis. The present study was designed to examine the subcellular events in the fetal adrenal that were apparently stimulated by estrogen-induced alterations in transplacental corticosteroid metabolism. Therefore, we determined the effects of estrogen treatment at midgestation and removal of estrogen action near term on the specific activity of the rate-limiting enzymes delta 5-3 beta-hydroxysteroid dehydrogenase (3 beta HSD) and 17-hydroxylase-17,20-lyase (17 alpha-OHase). Fetal adrenals were obtained on day 100 (n = 11) or day 165 (n = 11) of gestation (term = day 184) from untreated animals, on day 100 from animals receiving delta 4 A daily between days 70-100 (n = 9) to increase placental estrogen production, and on day 165 from baboons treated daily between days 130-164 with antiestrogen ethamoxytriphetol (MER-25; n = 7). The activity of 17 alpha-OHase was determined by incubating adrenal microsomes (105,000 x g) with [3H] progesterone, NAD+, and NADH in phosphate buffer. The radiolabeled products 17-hydroxyprogesterone, delta 4 A, and testosterone were purified, and enzyme activity expressed as picograms of product per min/mg tissue. The activity of 3 beta HSD was determined by incubating adrenal microsomes with [3H]pregnenolone and NAD+ in phosphate buffer. The radiolabeled progesterone product was purified, and enzyme activity was expressed as nanograms per min/mg tissue. Treatment with delta 4 A increased estrogen concentration at midgestation 3-fold to levels comparable to those measured near term. Although fetal adrenal weight was greater at term than at midgestation (p less than 0.05), weight was not increased by delta 4 A treatment. The specific activity (mean +/- SE) of fetal adrenal 17 alpha-OHase at midgestation (181 +/- 29) was increased (P less than 0.05) 3-fold by treatment with delta 4 A to levels (591 +/- 105) comparable to those in adrenal microsomes prepared from untreated animals near term (816 +/- 130). Enzyme activity in adrenals of MER-25-treated baboons was 40%, but not significantly lower than that in term controls.(ABSTRACT TRUNCATED AT 400 WORDS)