Clearance of a parenchymal injection of xenon Xe 133 from the jejunum was used to asses changes in tissue perfusion produced by variations in superiorr mesenteric artery flow resulting from partial aortic occlusion. Disappearance of xenon from submucosa and muscularis was similar and reproducible. The biexponential function of the isotope clearanc exhibited a rapid initial component representing mean flow. Calculated xenon clearance rates, expressed as half-times for isotope disappearance and plotted as a function of decreasing superior mesenteric artery flow, were characteristically rapid for a broad range of superio mesenteric artery flows (90 to 600 ml/min). With reduction of superior mesenteric artery flow beyond 80 plus or minus 10 ml/min, tissue clearance of xenon was markedly prolonged. Adequate perfusion of the vascular compartments of the small bowel as measured by xenon clearance was maintained until 80% reduction of superior mesenteric artery flow.