Plasma cortisol and corticosteroid-binding globulin (CBG) levels were assessed in pregnant squirrel monkeys and in intact and castrated males after estrogen administration. Pregnant females showed a rapid and dramatic rise in cortisol and CBG during the first 8 weeks after conception. Estrogen treatment also caused marked elevations in cortisol and CBG. Cortisol levels increased significantly by 24 h after estrogen injection and remained elevated for 6 weeks of treatment, but a relatively greater rise in CBG resulted in a higher CBG/cortisol ratio. The data support prior research indicating that estrogen can simultaneously stimulate adrenal output and the compensatory binding of circulating cortisol by increased CBG synthesis. In addition, it appears that even in the absence of exogenous treatment, the pituitary-adrenal axis of male squirrel monkeys is stimulated by estrogen derived either from the testes or by the peripheral conversion of testosterone to estrogen.