The present study was designed to determine whether PRL, in addition to ACTH, stimulates adrenal secretion of dehydroepiandrosterone (DHA) in vivo at midgestation in the baboon fetus (Papio anubis). On day 100 of gestation (term = day 184), fetuses were exteriorized, and a constant infusion of saline (0.1 ml/min) was initiated via a fetal femoral vein. Forty minutes later, a bolus injection of 30 nmol ACTH/ml saline (n = 5), 40 nmol ovine PRL/ml saline (n = 4), or 1 ml saline (n = 5) was administered via the fetal femoral venous catheter. ACTH (0.15 nmol/min.0.1 ml saline), PRL (0.20 nmol/min.0.1 ml saline), or saline (0.1 ml/min) was then infused for an additional 25 min. Blood samples were obtained from the contralateral fetal femoral vein and the maternal saphenous vein immediately before and after peptide infusion and from the umbilical vein and artery at the end of the infusion. Fetal serum DHA concentrations (range, 9-11 micrograms/100 ml) were significantly increased (P less than 0.05) by PRL and ACTH, but not by saline. In contrast, fetal concentrations of cortisol (15-20 micrograms/100 ml) and DHA sulfate (DHAS; 13-18 micrograms/100 ml) were not altered by infusion of test substances into the fetus. The maternal concentrations of F (49-61 micrograms/100 ml) and DHAS (19-22 micrograms/100 ml) exceeded (P less than 0.05) respective values in the fetus, whereas DHA concentrations (2-3 micrograms/100 ml) in the mother were lower (P less than 0.05) than those in fetal serum. The serum concentrations of DHA, DHAS, and cortisol in the mother were not altered by PRL or ACTH. Regardless of the treatment, concentrations of DHA and DHAS in umbilical vein were lower (P less than 0.05) than those in the umbilical artery. These findings indicate that PRL as well as ACTH are effective in vivo in stimulating serum DHA concentrations in fetal baboons at midgestation. The greater concentration of DHA in umbilical artery vs. umbilical vein as well as the lack of response in maternal DHA concentrations indicate that the site of action of PRL and ACTH is the fetal adrenal. Therefore, we conclude that at midgestation, there is the potential for multifactorial regulation of baboon fetal adrenal androgen production and that PRL, in addition to ACTH, can function as a fetal adrenocorticotropic factor in vivo.