Neonatal human males produce high levels of dehydroepiandrosterone (DHEA) and its sulfo-conjugated form (DS) that decline within a few months of birth, due to regression of the adrenal fetal zone (FZ). Adult male humans and rhesus monkeys produce C19 steroids in abundance from the adrenal zona reticularis (ZR). Male marmoset monkeys produce DS at birth, but unlike humans and rhesus monkeys, do not produce comparable amounts of DHEA and DS in adulthood. To determine whether male marmosets express a functional ZR in adulthood, we examined adult and neonatal male marmosets for the presence of a ZR and FZ, respectively. Exogenous ACTH failed to stimulate DHEA or DS in adults, and dexamethasone treatment failed to suppress DHEA and DS, although cortisol levels changed as expected. In steroidogenic tissues, the key proteins necessary to synthesize C19 steroids from pregnenolone are P450c17, 3-hydroxysteroid dehydrogenase (3-HSD), NADPH oxido-reductase cytochrome P450 (reductase), and cytochrome_b5 (cyt_b5). Adult adrenal cross-sections showed P450c17 and reductase protein expression throughout the cortex, but showed no expected decrease in 3-HSD and increase in cyt_b5 in the innermost region. Western analysis confirmed these data, demonstrating comparable P450c17 expression to rhesus monkeys, but not cyt_b5. HPLC analysis revealed similar 17-hydroxylase action on pregnenolone for adult marmoset and rhesus adrenal microsomes, but greatly diminished 17,20-lyase activity in marmosets. Neonatal marmoset adrenals exhibited staining indicative of a putative FZ (with P450c17, reduced 3-HSD and increased cyt_b5). We conclude that neonatal marmosets exhibit a C19 steroid-secreting FZ similar to humans, but adult males fail to acquire a functional ZR.