Hemorrhage is a stress on the cardiovascular system that results in decreased loading of the heart but also decreased blood pressure and thus decreased perfusion pressure for tissue blood flow. The heart's response to hemorrhage is governed by both an increase in sympathetic nervous system activation of the heart and decreased preload and afterload for the heart. Whether the heart can maintain normal contractile function and reserves under conditions of prolonged hemorrhagic shock is not clear. To assess the effects of hemorrhagic shock of different lengths on intrinsic cardiac contractile function, guinea pigs were surgically prepared for the measurement of blood pressure, heart rate, and cardiac output and blood samples were taken for the measurement of metabolic indices of cardiovascular stress. Fifty percent of the animals' blood volume was removed and then animals were followed for 1, 2, or 3 h of hemorrhagic shock. Hearts were then removed for measurement of intrinsic contractile function. Hearts from animals exposed to 1 or 2 h of shock exhibited normal ventricular function although hearts removed after 3 h exhibited changes in ventricular function. Maintenance of normal cardiac function through at least 2 h of shock must represent adequate physiologic modulation of coronary blood flow to deliver adequate oxygen to match the myocardial oxygen demands under conditions of severe blood loss. This balance may be disrupted by 3 h of shock thus resulting in loss of contractile reserve.