Irreversible hemorrhagic shock in dogs was associated with a progressive rise in plasma hemoglobin levels and the appearance of an abnormal hemin pigment in the plasma. Both findings were related to mucosal necrosis of the bowel occurring as a result of prolonged hemorrhagic shock. In dogs perfused through the superior mesenteric artery during hemorrhagic shock, the bowel remained normal, irreversibility was prevented and the abnormal plasma findings did not occur.