The primate fetal adrenal reaches a large size relative to body weight followed by a rapid decrease in size in the postnatal period. We tested the hypothesis that maternal melatonin stimulates growth and prevents maturation of the primate fetal adrenal gland. We suppressed maternal melatonin by exposing eight pregnant capuchin monkeys to constant light (LL) from 63% to 90% gestation (term 155 days). Three of these received daily oral melatonin replacement (LL + Mel). Five mothers remaining in light:dark cycle were used as controls. Fetuses were delivered at 90% gestation. The absence of maternal melatonin selectively decreased fetal adrenal weight (Control: 488.8 ± 51.5; LL: 363.2 ± 27.7 and LL + Mel 519 ± 46 mg; P < 0.05 ANOVA) without effecting fetal weight, placental weight or the weight of other fetal tissues. Changes in fetal adrenal size were accompanied by an increase in the levels of Δ5-3β-hydroxysteroid dehydrogenase (3β-HSD) mRNA (Control: 0.8 ± 0.2; LL: 5.2 ± 0.6 and LL + Mel 0.8 ± 0.1; 3β-HSD/18S-rRNA; P < 0.05 ANOVA). In vitro we found that maternal melatonin suppression increased basal progesterone production to levels similar to those of the adult adrenal gland (Control: 0.36 ± 0.09; LL 0.99 ± 0.13; LL + Mel 0.18 ± 0.06 and adult: 0.88 ± 0.10 ng/mg of tissue; P < 0.05 ANOVA) but no change in cortisol production. We found an increased production of cortisone (Control: 1.65 ± 0.60; LL: 5.44 ± 0.63; LL + Mel: 2.90 ± 0.38 and adult: 1.70 ± 0.45 ng/mg of tissue; P < 0.05 ANOVA). Collectively, the effects of maternal melatonin suppression and their reversion by maternal melatonin replacement suggest that maternal melatonin stimulates growth and prevents maturation of the capuchin monkey fetal adrenal gland.