The effects of thermal injury on circulation, respiration, and complement activity was studied in dogs. An aggregation of platelets in the blood and trapping in the lung were observed following burn injury. Leukopenia appeared first, then leukocytosis. Hemolysis of red cells corresponding to a destruction of about 30% of the erythrocytes was observed. A blood pressure decrease and a tracheal insufflation pressure increase followed thermal injury. Blood gas and lactate changes like those seen during a low perfusion state were recorded, but signs of impaired oxygenation of arterial blood were also observed. A possible mechanism by which denatured proteins might initiate some of the symptoms seen after thermal injury is suggested and its relation to complement activation is evaluated.