Thymosin-alpha 1 (thymosin alpha 1) and cortisol levels were evaluated in juvenile squirrel monkeys to investigate the influence of psychological disturbance on thymic and adrenal hormone activity. Hormone levels were assessed in peripheral circulation following removal of monkeys from their social group to establish the time course of thymosin and cortisol alterations. Thymosin alpha 1 was significantly decreased after social separation in association with increased adrenocortical activity, especially during the first day after being housed alone. The temporal pattern suggested that both hormone systems are involved in the acute inhibition of functional immunity observed following this type of psychological disturbance. A second study verified that the decrement in thymosin alpha 1 levels was replicable and also sensitive to psychosocial factors that influence the level of induced disturbance. In addition, changes in thymosin secretion could be attenuated partially by pharmacological inhibition of the cortisol response and opiate hormone action. Nevertheless, the decrease in thymosin alpha 1 secretion did not appear to be a secondary consequence of adrenocortical secretion, and instead probably emanates from a general shift in neuroendocrine activity.