Pituitary-adrenal responsiveness to stress induced by either psychological (novelty) or physiological (ether) stimuli was examined in young (3 to 6 months) and old (24 to 27 months) male and female rats. In Experiment 1, subjects were placed in a novel environment for 3 min. Blood samples were collected at 0, 15, 30, 45, and 60 min. In Experiment 2, a blood sample was collected from experimental subjects immediately following 15 min in a novel environment; a basal sample was collected from control subjects. In Experiment 3, basal and stress blood samples were collected from subjects exposed to ether vapors. Blood samples were assayed for plasma corticosterone content. The major findings were: (1) no change with age in either sex in basal corticosterone levels or in the time course of the adrenocortical response; (2) no change with age in males in stress-induced increments in corticoids; (3) no change with age in females in corticoid elevations induced by a mild stressor (Experiment 1), but a decrement in elevations induced by more potent stressors (Experiments 2 and 3).