It is well-known that the cortisone requirement of an Addisonian patient increases during an acute illness. The physiological function of increased circulating levels of hydrocortisone during a stressful situation has been the subject of considerable controversy. In an endeavor to explain the clinical usefulness of increased adrenocortical hormone during a major illness, adrenalectomized dogs were studied following surgical trauma or hemorrhage. Plasma volume, arterial blood pressure, and hematocrit reading were measured before and at varying intervals after the experimental stress. Water and food were withheld for the entire period of observation. Within 18 hours following minor operative trauma or a