The partition of Li+, Br-, and I- across the membrane of the sartorius muscle of the toad Bufo marinus has been investigated both at the steady state and with kinetic methods. Li+ was found to have access to an amount of muscle water similar to that of Na+. Br- and I- could be regarded as being interchangeable with cellular Cl-. None of the foreign ions caused significant losses of cellular K+.

Li+ efflux from the cell was slower in muscles which were equilibrated for long periods in Li+ than in short equilibrated muscles. Na+ efflux from Li+-treated muscles was similar in rate to normal controls, but the amount of Na+ in the slow fraction was increased by Li+.

I- efflux was extremely rapid, and it was not possible to differentiate kinetically between intra- and extracellular material.

These results have been found to be consistent with the hypothesis of a three phase system for muscle.

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