To assess the effects of external counterpressure in a hypovolemic canine model, mean right atrial pressure (RA), left ventricle end-diastolic pressure (L VEDP), mean aortic pressure (Ao), and cardi'ac output (CO) (indicator dilution technique or electromagnetic ascending aortic flow) were measured in eight closed-chest mongrel dogs following phlebotomy to an Ao of 50 to 60 mm Hg. Inferior vena cava (IVC) flow was measured electromagneticalIy with a carmulating probe in four animals. The antishock garment was mflared to pressures of 40, 60, 80, and 100 mm Hg. An extended shock "'control" period preceded inflation to minimize the effect of reflex circulatory responses to acute blood loss. IVC flow (2 +-1 mL/min/kg) during and immediately following antishock garment inflation was not significantly different from control (3 +-_ 1 mL/min/kg) regardless of inflation pressure. Ao, RA, and LVEDP measured 30 seconds and 15 minutes after garment inflation were increased, but CO was not significantly different from control values at each inflation pressure. Garment inflation significantly increased peripheral vascular resistance (PVR) at all inflation pressures, and there was a significant correlation (r = .53; P < .001) between the change in Ao and PVR. These results indicate that the change in arterial pressure produced by external counterpressure is the result of an increase in PVR and not the result of an autotransfusion and subsequent increased left heart outflow in the canine shock model.