The serum concentrations of progesterone and 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OHP) were measured by RIA in blood draining the maternal and fetal sides of the placenta on Day 100 and Day 170 of gestation (term = Day 184) in the baboon to determine the qualitative and quantitative patterns of progestins within the maternal-placental-fetal compartment and to ascertain whether production of placental progestins is increased with advancing gestation. The mean (+/- SEM) concentration of progesterone in maternal serum was similar at mid- (10 +/- 2 ng/ml) and late (11 +/- 3 ng/ml) gestation and not different than that of 20 alpha-OHP (6 +/- 1 ng/ml). In the uterine vein, progesterone levels were greater (p < 0.05) at Day 100 (82 +/- 13 ng/ml) than at Day 170 (30 +/- 7 ng/ml) and exceeded (p < 0.05) those of 20 alpha-OHP at both mid- (8 +/- 2 ng/ml) and late (4 +/- 1 ng/ml) gestation. Progesterone concentrations in the umbilical vein (128 +/- 24 ng/ml) and artery (79 +/- 12 ng/ml) at midgestation exceeded respective values at term (20 +/- 1 and 14 +/- 1 ng/ml). In contrast, 20 alpha-OHP concentrations in the umbilical vein (17 +/- 4 ng/ml) and artery (12 +/- 3 ng/ml) at midgestation increased more than 2-fold by Day 170. The estimated secretion rate of placental progesterone into the fetus was similar at mid- (752 +/- 154 ng/min) and late (681 +/- 171 ng/min) gestation, whereas that for 20 alpha-OHP was negligible at midgestation (57 +/- 71 ng/min) and increased 15-fold (p < 0.05) by term (892 +/- 241 ng/min). Because 20 alpha-OHP is a metabolite of progesterone, total placental progesterone production was greater (p < 0.05) at term (1595 +/- 400 ng/min) than at midgestation (809 +/- 171 ng/min). This study demonstrates that placental secretion of progesterone is bidirectional whereas that of 20 alpha-OHP occurs selectively into the fetus. Although progesterone and 20 alpha-OHP levels in the fetus were lower at term than at midgestation, because of the developmental increase in umbilical blood flow as determined by others, placental progesterone production was actually increased during this interval. Therefore, we suggest that the estrogen-dependent developmental increase in key components of the progesterone biosynthetic pathway, recently demonstrated by us in the baboon placenta, is associated with a corresponding increase in progesterone production.