Rhythmical changes in small vein pressure were shown to follow the direction of changes in intra-abdominal pressure. Pressure waves in small veins could be diminished considerably during femoral artery occlusion when the venous cannula was directed away from the vena cava. They were only slightly affected when the venous cannula was directed toward the vena cava. Small vein pressure waves were markedly decreased by opening the abdomen and reducing intra-abdominal pressure.It is concluded that intra-abdominal pressure change is a major factor in varying small vein pressure in the hindleg of an anesthetized dog in a supine position.