The blood pressure, heart rate, ACTH, corticosteroid, vasopressin, and renin responses to rapid 15 ml/kg hemorrhage were measured in six conscious healthy dogs with chronically maintained femoral arterial catheters. The hemorrhage decreased the mean arterial blood pressure slightly (P less than 0.001), increased the heart rate (P less than 0.001), and increased arterial plasma levels of ACTH (P less than 0.01), corticosteroids (P less than 0.001), vasopressin (P less than 0.001), and renin activity (P less than 0.001). Overall and in the individual experiments, there appeared to be little correspondence between the ACTH and corticosteroid responses. In none of the experiments was there a clear rise in ACTH above control levels before the first rise in corticosteroids. To ascertain that adrenal secretion of corticosteroids was increased during 15 ml/kg hypovolemia, changes in the clearance and distribution volume of cortisol were estimated by counting tritium extracted from plasma of five dogs infused with [1,2-3H] cortisol to steady state levels before and during hypovolemia. The stimulus caused a 30% reduction from steady state levels of dichloromethane-extractable tritium counts (P less than 0.001). Combined with the observed increase in plasma corticosteroid levels, these results show that the increase in adrenal secretion of corticosteroids after hemorrhage was underestimated by measurement of changes in peripheral plasma levels. The hypothesis that hemorrhage results in an increase in adrenal sensitivity to ACTH is tested in the following paper.