The developmental expressions of cytochrome b5 (b5), 17 alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17), and 3 beta-hydroxysteroid dehydrogenase were examined in primate fetal adrenals by immunocytochemistry from 50-160 d gestation. The expression of b5 was evident at 50 d in the developing fetal zone (FZ), but decreased markedly through midgestation, then increased again from 150 d to term. Similar changes in the temporal expression was observed for P450c17. Whereas P450c17 was induced largely in the transitional zone (TZ; outer-most FZ), b5 expression was strongest in FZ cells further from the capsule, although overlap between these regions involved a narrow band of cells beneath the TZ that may represent the developing zona reticularis. Thus, the induction of b5 in the FZ and of P450c17 in the TZ of the fetal adrenal late in gestation coincided temporally with the prepartum rise in dehydroepiandrosterone previously reported. These data are consistent with the proposed role of b5 in supporting 17,20-lyase activity of P450c17. However, the lack of cytochrome b5 and P450c17 expression in the FZ of the developing macaque adrenal cortex for much of the second and third trimesters distinguishes it from the mature zona reticularis seen in adult animals.