The present study determined whether the elevation in oestrogen, which occurs with advancing baboon pregnancy, is associated with a developmental increase in expression of the placental enzymes catalysing progesterone synthesis. The mRNA levels for P-450 cholesterol side-chain cleavage (P-450scc), adrenodoxin, and delta 5-3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD) were assessed by Northern blot analysis in placental syncytiotrophoblasts isolated from baboons in early (days 58-65), mid- (days 97-113) and late (days 161-175), gestation (term = 184 days). Placental villous tissue was dispersed and subjected to 50 per cent Percoll density gradient centrifugation to obtain primarily syncytiotrophoblasts. Mean (+/- S.E.) P-450scc mRNA level, expressed as a ratio of beta-actin in the syncytiotrophoblast-rich fraction, progressively increased with advancing pregnancy to a level in late gestation (1.81 +/- 0.28 arbitrary units) that was approximately sixfold (P < 0.01) greater than in early gestation (0.31 +/- 0.08) and approximately twofold greater (P < 0.05) than in mid-gestation (0.97 +/- 0.24). In contrast adrenodoxin mRNA expression was similar at early (0.97), mid- (1.14 +/- 0.12) and late (1.16 +/- 0.13) gestation. Syncytiotrophoblast 3 beta-HSD mRNA levels also remained constant in early (1.69), mid- (1.89 +/- 0.41) and late (1.34 +/- 0.41) gestation. On the basis of these findings, we propose that villous syncytiotrophoblasts undergo functional/biochemical differentiation, resulting in a coordinated upregulation of specific components of the steroid biosynthetic pathway required for progesterone biosynthesis during the course of primate pregnancy.