Throughout the majority of intrauterine development, the primate fetal adrenal gland is comprised primarily of fetal zone cells and only late in gestation do definitive zone cells, which express the enzyme delta5-3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) emerge to produce cortisol. The present study was designed to determine whether the induction of definitive zone ACTH receptor messenger RNA (mRNA) levels and components of the steroidogenic pathway known to be expressed specifically in the definitive zone, e.g. the 3beta-HSD enzyme, are dependent upon fetal pituitary ACTH. Fetal pituitaries and adrenal glands were obtained on day 165 (term = day 184) from untreated controls (n = 7) and from baboons in which betamethasone was administered im to the fetus (0.6 mg/100 microl; n = 4) or to the fetus (0.6 mg) and mother (6 mg/ml; n = 4) every other day between days 150 and 164 of gestation. Although fetal pituitary weight was not altered by betamethasone, POMC mRNA levels determined by in situ hybridization were lower (P < 0.05) in betamethasone-treated (0.34 +/- 0.07 arbitrary densitometric units) than in untreated controls (0.63 +/- 0.04). Associated with this decline in pituitary POMC, levels of the major 3.4-kb mRNA transcript for the ACTH receptor expressed as a ratio of beta-actin were approximately 80% lower (P < 0.05) in fetal adrenals of betamethasone-treated baboons (0.12 +/- 0.02) than in untreated controls (0.84 +/- 0.05). In situ hybridization indicated that ACTH receptor mRNA expression in the definitive zone exceeded that in the fetal zone and was reduced by betamethasone. Associated with the decrease in ACTH receptor expression, fetal adrenal weight was suppressed (P < 0.05) by 50% and reflected a marked reduction (P < 0.05) in the size of the cells of the definitive and fetal zones. Betamethasone treatment also induced a decrease (P < 0.05) in the width (microm) of the definitive zone (183 +/- 14 vs. 128 +/- 7; determined by immunohistochemical expression of 3beta-HSD), as well as the levels of the mRNA and protein for 3beta-HSD. Levels of the mRNA for the LDL-receptor and the enzymes 17alpha-hydroxylase-C(17,20) lyase and P450 cholesterol side chain cleavage were also suppressed in adrenals of betamethasone-treated baboons. These findings indicate that treatment of the baboon fetus with betamethasone in late gestation suppressed fetal pituitary POMC mRNA expression and ACTH receptor mRNA levels in the fetal adrenal gland, as well as the hypertrophy and ACTH receptor mRNA and 3beta-HSD mRNA/protein levels in the cells comprising the newly emerging definitive zone. We conclude that ACTH is necessary for the up-regulation of the mRNAs for the ACTH receptor and steroidogenic enzymes in the definitive zone of the primate fetal adrenal gland in late gestation.