We have hypothesized that the change in placental cortisol (F)-cortisone (E) metabolism induced by estrogen late in gestation is important to activation of the baboon fetal hypothalamic-pituitary-adrenocortical axis, culminating in the ontogenesis of de novo F secretion by the fetal adrenal. The present study tested this hypothesis in vivo by comparing the proportion of F in the fetus derived via maternal and fetal production on day 100 (n = 7; term = day 184) and day 165 (n = 4) in untreated baboons and on day 100 in baboons (n = 9) in which 50-mg pellets of androstenedione were implanted sc in the mother in increasing numbers (i.e. two on day 70, four on day 78, six on day 86, and eight on day 94) to increase placental estrogen production. Maternal, uterine, and umbilical venous samples were collected during constant maternal infusion (120 min) of [3H]F/[14C]E, endogenous and radiolabeled F/E content was determined, and corticosteroid dynamics were quantified. The MCR and peripheral interconversion of F and E as well as the production rate of F were unaltered in the mother. However, at midgestation, androstenedione increased (P less than 0.05) estrogen by 62% and altered transuterofeto placental F-E metabolism from preferential reduction of E (%F----E = 17 +/- 4; %E----F = 27 +/- 7) to preferential oxidation of F (%F----E = 26 +/- 2; %E----F = 5 +/- 2), a pattern similar to that at term (%F----E = 32 +/- 6; %E----F = 9 +/- 4). In untreated baboons, on day 100 none of the F in the fetus was due to fetal production, whereas by day 165, 49 +/- 6% was of fetal origin. In animals treated with androstenedione at midgestation, 22 +/- 4% of fetal F was derived de novo within the fetus. Thus, production of F by the fetus was negligible on day 100, increased near term in association with an increase in transplacental oxidation of F to E, and was induced at midgestation in baboons in which placental F-E metabolism was altered by an increase in estrogen production. These findings provide indirect evidence that supports our hypothesis that the change in placental F-E metabolism induced by increasing estrogen late in pregnancy results in activation of the fetal hypothalamic-pituitary-adrenocortical axis and, thus, ontogenesis of fetal F production near term.