Values of unidirectional Na fluxes measured across the vascularly perfused small intestine of three anuran species are higher than those found in other preparations in vitro. In Rana ridibunda and R. pipiens no net movement of Na across the small intestine can be detected. In contrast, the unidirectional fluxes of Na across the colon of R. ridibunda and R. pipiens are lower than across the small intestine and a significant net absorption is found. Apparent loading and unloading pools for Na within the small intestine, as measured with tracer Na under standard experimental conditions, consist largely of extracellular Na presumably within the bulk phase of the lumen. The size of these pools can be greatly reduced by the rapid addition to or removal from the lumen of tracer. The loading pool appears to occupy not more than about 9% of the total tissue water, equivalent to about 20% of the extracellular water of the tissue. The washout of tracer Na from preloaded small intestine into the vascular bed is bi-exponential and appears from a pool, or pools, of apparent greater size than that of the loading pool. The results show that Na can move very rapidly across the small intestine and suggest that a high proportion of this movement occurs possibly via paracellular shunt pathways.