Dehydroepiandrosterone (D) and dehydroepiandrosterone sulphate (DS) concentrations were determined by radioimmunoassay in peripheral sera of non-pregnant, pregnant (55 days to term) and newborn baboons and in umbilical sera of animals delivered by Caesarean section close to term. D concentrations (mean +/- SD, microng/100 ml, N) in non-pregnany animals (1.61 +/- 1.32, 23) were not different (P greater than 0.05) from those during pregnancy (1.80 +/- 1.21, 101). DS concentrations, expressed as unconjugated D, in non-pregnant (13.5 +/- 6.0, 23) and pregnant (15.1 +/- 7.5, 101) animals were also similar (P greater than 0.05). However, both D (P less than 0.01) and DS (P less than 0.005) levels increased with gestational age such that serum D (2.46 +/- 1.39, 23, P less than 0.05) and DS (18.9 +/- 5.7, 23, P less than 0.001) levels between 150 and 180 days gestation were greater than in non-pregnant animals. These increases may be important since oestrogen production rises rapidly during late gestation and both D and DS can serve as oestrogen precursors. In both non-pregnant (P less than 0.005) and pregnant (P less than 0.001) animals D and DS concentrations (ratio 1:8) were correlated. Cord serum D levels (2.4 +/- 1.4, 5) were not different from those of maternal serum (P greater than 0.05), while cord DS levels (40.3 +/- 14.8) were greater (P less than 0.001) than those of maternal serum. This may reflect rapid equilibration of D but not DS between foetal and maternal circulations. In sera from neonates, D (19.4 +/- 14.6, 8) and DS 567 +/- 570, 8) concentrations were greater (P less than 0.001) than those in maternal serum, indicating marked post-partal changes in clearance or production of both compounds. The high levels of D and DS in cord and newborn wera are compatible with the view that the baboon foetus makes appreciable contributions of oestrogen precurosrs in pregnancy.