Adrenal secretory rates of cortisol and corticosterone and arterial concentrations of ACTH and cortisol were measured in conscious trained dogs subjected to 10 ml/(kg . 3 min) or 20 ml/(kg . 3 min) hemorrhage. All four variables increased substantially after 20 ml/(kg . 3 min) hemorrhage. Secretion rates of cortisol and corticosterone increased significantly after 10 ml/(kg . 3 min) hemorrhage, without a change in ACTH. The responses of ACTH and the secretion rates of cortisol to 10 ml/(kg . 3 min) hemorrhage and iv infusion of ACTH were compared. Infusions of ACTH required to match the secretory response of cortisol after 10 ml/(kg . 3 min) hemorrhage resulted in concentrations of ACTH significantly higher than those observed after 10 ml/(kg . 3 min) hemorrhage. These results suggest that 10 ml/(kg . 3 min) hemorrhage induces an acute increase in adrenocortical sensitivity to ACTH.