Methods Ten large adult mongrel dogs were anes-thetized with intravenous pentobarbital sodium, ventilated with a mechanical respirator, andplaced in a pressure garment en-closing the hindquarters (Fig. 1). After a splenectomy was performed, all aortic branches between the renal arteries and the aortic trifurcation were ligated and divided, and both inferior epigastric arteries were ligated. Brachial artery and dorsalis pedis artery pressures were recorded through pressure transducers on a direct writing oscillograph. Arterial blood pH was meas-ured with a Beckman Research Model pH meter. Cardiac output was estimated using radioactive iodinated serum albumin and Iodinel3l tagged diodrast after the method of Huff. 8 Blood volumes were determined with radioactive iodinated serum albumin and periodic arterial hematocrits were ob-tained. All measurementswere done before, during, and after 60 minutes of infrarenal aortic crossclamping. After a period of stabilization, the same ten dogs were subjected to another 60 minutes of infrarenal aortic crossclamping, but this time, im-mediately prior to clamping the aorta, the counterpressure garment was inflated with air until pulse pressure in the dorsalis pedis artery was obliterated. Pneumatic counter-pressure was maintained during the entire hour of aortic clamping and was gradually reduced over a ten minute period immediately following release of the aortic clamps. 614