Reece1 has stated that adrenocortical insufficiency rarely results from surgical trauma. He further states that adrenocortical response is related to the magnitude of the stress and the ability of the adrenals to respond. In open-heart surgery employing total body perfusion, the complete exchange of blood volume imposes an unusual stress on the human organism and the adrenal cortex. Also, suboptimal blood flow to the adrenals, occurring during the cardiopulmonary bypass, might compromise their secretory reserve. Adrenal insufficiency was suspected early in the open-heart surgery program because an occasional patient developed hyperthermia and hypotension immediately after surgery. The hyperthermia was unresponsive to oral and intravenous salicylates and surface cooling and the hypotension did not respond to the usual measures. However, both were controlled promptly with the intravenous administration of 100-200 mg. of cortisone. Therefore, this study of 103 cases was undertaken to determine the adrenocortical response following total body perfusion