The hormonal reaction of adrenal and gonadal glands of baboons at various ages was studied under 2 hr immobilization stress condition. Concentrations of cortisol, 11-deoxycortisol, 17-hydroxypregnenolone, dehydroepiandrosterone and testosterone were determined by radioimmunoassay in the monkey blood plasma at different times during the stress reaction. A more pronounced reaction of adrenal cortex was shown in 1 year old baboons. The peak of cortisol level in immature monkeys under stress conditions was registered much earlier than in adult monkeys.