This Paper is concerned with the pattern of cardiorespiratory and renal changes produced by opening a systemic arteriovenous fistula. Flow through bilateral femoral arteriovenous fistulas averaged approximately 53 per cent of the control cardiac output. The rise in cardiac output elicited by the shunt averaged 51 per cent of the shunt flow PAH clearances decreased 23 per cent in animals anesthetized with sodium pentobarbital, and 46 per cent in animals premedicated with morphine sulfate prior to administration of sodium pentobarbital. The per cent reduction in renal blood flow did not differ significantly from the per cent reduction in flow through the extrarenal systemic capillary beds.