The effects of surgical trauma and hemorrhagic shock on the circadian rhythmicity of corticosteroid secretion in the rat were investigated. The estimations were performed at 4-hour intervals for a 24-hour period. Control animals exhibited a characteristic circadian rhythm of plasma corticosterone with peak concentrations occurring at 8 p.m. followed by a gradual fall during the night, reaching a minimum at 8 a.m. Severe stress induced by hemorrhagic shock or surgical trauma caused a dramatic alteration in corticosterone rhythms which persisted up to 72 h following surgery or hemorrhage. It is apparent that the physiological mechanisms which regulate adrenal rhythmicity are disrupted for a prolonged period following major stress or trauma.