1. At a concentration of 2·5 mM in the external solution, Cs ions reduced K efflux in muscles incubated in Na media. This effect was demonstrated in the presence or absence of 2·5 mM-K and in the presence or absence of 10-4 M ouabain.2. In Ringer solution in which NaCl was replaced by an osmotic equivalent of MgCl2, external Cs ions increased K efflux if the solution was K-free and decreased K efflux if the solution contained 2·5 mM-K.3. External Cs ions reduced the inward rate of leakage of Na ions into muscle cells by about 25% when the medium was K-free and contained 10-5 M ouabain.4. The effects of 2·5 mM-K ions and 2·5 mM-Cs ions on the muscle fibre membrane potential were about the same. The influence of Cs ions on K efflux cannot be explained by changes in the resting membrane potential.5. The results suggest that a large part of the K efflux from muscle cells is mediated by a K:K exchange mechanism that is inhibited by external Cs ions.6. The results also suggest that a smaller part of the K efflux is due to K:Na exchange that is also inhibited by external Cs ions.7. In the absence of either external K or Na ions for internal K to exchange with, Cs ions promote a small amount of K exchange, perhaps via both mechanisms.
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