An abnormal communication between the peripheral arterial and venous systems produces typical hemodynamic effects of increased cardiac output, hypervolemia, and eventual heart failure. These changes are not seen following a splanchnic arteriovenous fistula. Cardiac output, measured before and after closure of a splanchnic arteriovenous fistula in one patient, showed no significant change. The absence of hemodynamic effects in this situation has been attributed to the resistance of the liver positioned between the heart and the fistula. However, experimental proof of this assumed "hepatic protection" is lacking. This investigation was designed to study the effect of interposing the liver between the heart and a peripheral arteriovenous fistula. Methods and Materials Fifteen mongrel dogs were used in this experiment. After an overnight fast, they were anesthetized with intravenously administered thiopental sodium and placed on a piston-type mechanical respirator. Hartmann's solution was infused intravenously at a rate of 50 ml/hr except when additional