This study describes the morphology of the squirrel monkey (Saimiri sciureus) adrenal gland in the perinatal period. Adrenals of fetal monkeys had a broad central zone of large eosinophilic cells (fetal zone) surrounded by a subcapsular band of small dark cells (definitive zone). Adrenals of stillborn and neonatal monkeys had a reduced fetal zone and an expanded definitive zone that was differentiated into distinct zona fasciculata and zona glomerulosa. By day 18 postpartum, no remnant of the fetal zone remained.