The outflux of barbital from the blood into the small intestine perfused with an isosmotic buffer of pH 9.5 in anesthetized rats was measured to determine the subepithelial capillary blood flow in the gut. It was shown that barbital clearance is blood flow limited when total blood flow to the small intestine varied between 0.3 and 1.3 ml/min per g wet gut. The barbital clearance amounted to an average of 50.3% of the total blood flow. The total mucosal blood flow determined by the use of the distribution of microspheres in the layers of gut wall was 62.76% of the total blood flow. It is concluded that, because of anatomical reasons, a subepithelial blood flow available for the transport process is somewhat less than the measured total mucosal blood flow.