1. Efflux curves of (24)Na, (42)K and (36)Cl from the guinea-pig taenia coli were obtained in normal Krebs solution, and in hypertonic Krebs solution in which the osmolarity had been doubled by the addition of sucrose.2. The efflux curves were complex, and in order to get average curves each was analysed as the sum of three exponential terms, and average curves were constructed from the means of the constants found.3. In order to estimate the membrane permeability to the ions, it was necessary to make assumptions as to the distribution of ions in the tissue. Several models have been examined and the predictions of the models with respect to the membrane properties were compared with the data obtained by using electrophysiological methods by other workers.4. It was found that reasonable predictions of membrane properties could only be made using models in which the majority of the rapidly exchanging sodium is considered to be extracellular. This amount of sodium is more than can be accounted for as freely dissolved in the extracellular water.5. A possible interpretation of the ion exchange and distribution would be to suppose that some proportion of the three ions is contained in an extracellular compartment not available to the normal extracellular markers, and limited in its exchange by the rate of diffusion in the extracellular phase, and that the truly intracellular fractions of the tissue ions do not exchange with the external solution in a simple exponential manner, but in a manner described by an aggregate of exponential terms due to inherent variation in the permeabilities of the individual cell membranes.6. There is no evidence for any change in the membrane permeabilities to sodium and potassium in hypertonic solution, but there is evidence for a decrease in chloride permeability in this solution.