1. A whole-gut perfusion technique has been used to compare the rates of intestinal absorption of sodium and xenon.
2. The calculated transit-time spectra for sodium and xenon across the gut mucosa do not differ significantly either in mean transit time or, for the first 15 min, in shape.
3. These results support the hypothesis that the rate of transfer of sodium from the small intestinal lumen to plasma is limited by blood flow.
4. It is suggested that some features of cholera can be explained by this hypothesis.