Although fetal pituitary ACTH is important to fetal adrenal growth and steroidogenesis in the second half of primate pregnancy, its role in adrenal development and function has not been established in vivo in the first half of gestation. In the present study, therefore, baboons were treated at midgestation with betamethasone to determine the effect of fetal pituitary ACTH on fetal adrenal growth, development, and ACTH receptor and P-450 enzyme messenger ribonucleic acid (mRNA) levels. The administration of betamethasone to baboon mothers on days 60-99 of gestation (term = 184 days) decreased fetal pituitary POMC mRNA levels by 54% (P < 0.01) and fetal serum ACTH levels to undetectable values (P < 0.05). The decline in ACTH was associated with decreases in fetal adrenal weight (P < 0.001), cortical cell size (P < 0.05), appearance of apoptosis and cellular disorganization, and a loss of immunocytochemically demonstrable definitive zone-specific delta5-3beta-hydroxysteroid dehydrogenase expression. The concomitant administration of ACTH and betamethasone restored these aspects of adrenal integrity to normal. Moreover, there was approximately a 95% decrease (P < 0.01) in fetal adrenal expression of ACTH receptor, P-450 cholesterol side-chain cleavage, and P-450 17alpha-hydroxylase 17/20-lyase mRNA levels after betamethasone administration. We conclude that fetal pituitary ACTH is necessary for the growth and development of fetal and definitive cortical zones and the marked coordinated increase in ACTH receptor and maintenance of P-450 cholesterol side-chain cleavage/P-450 17alpha-hydroxylase 17/20-lyase expression in the baboon fetal adrenal gland during the first half of gestation.