We have shown that ACTH receptor mRNA expression and steroidogenesis were increased in the transitional zone and decreased in the fetal zone of the baboon fetal adrenal in the second half of gestation. Thus, we proposed that there is a divergence in ACTH receptor-mediated zone-specific steroidogenesis within the fetal adrenal during mid to late gestation. We have also demonstrated that fetal serum alpha-inhibin levels decline with advancing development. It is possible, therefore, that the alpha subunit of inhibin provides a good marker of fetal zone cellular function and that the changes in circulating fetal alpha-inhibin with advancing pregnancy reflect ontogenetic changes in fetal adrenal cortical zone-specific cell function. However, it remains to be determined whether the fetal adrenal is a major source of circulating alpha-inhibin in the fetus and whether alpha-inhibin is expressed in the fetal, definitive, and/or transitional zones. Therefore, the current study compared fetal serum alpha-inhibin levels with immunocytochemical localization of alpha-inhibin in baboon fetal adrenals obtained on Days 60 (early), 100 (mid), and 165 or 182 (late) of gestation (term averages Day 184) from animals untreated or treated with betamethasone, which we previously demonstrated suppressed fetal pituitary ACTH and adrenal weight. Fetal serum alpha-inhibin levels (mean +/- SE) were greater (p < 0.05) at mid (5863 +/- 730 microliter eq/ml) than at late (3246 +/- 379) gestation and were reduced (p < 0. 05) by betamethasone. The inhibin alpha subunit was expressed in abundant quantities in the fetal adrenal cortex, but not in medulla, throughout gestation. At mid and late gestation, alpha-inhibin was expressed throughout the fetal adrenal cortex but most intensely in the innermost area of fetal zone cells. By late gestation, the fetal adrenal exhibited a gradient of alpha-inhibin expression. Thus, the outermost definitive zone cells were devoid of alpha-inhibin, the transitional zone exhibited a relatively low alpha-inhibin content, and fetal zone cells continued to exhibit extensive expression of alpha-inhibin. Betamethasone diminished the intensity of alpha-inhibin expression throughout the fetal adrenal cortex. These results indicate that the fetal adrenal fetal zone is a significant source of circulating alpha-inhibin in the baboon fetus and that alpha-inhibin provides a good marker to study the developmental regulation of fetal zone-specific adrenocortical function.