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1. The kinetics of Na efflux were studied in oocytes of Bufo bufo, Rana temporaria and R. pipiens. 2. Rate constants for Na efflux into Ringer solution varied from 0-002 min-minus 1 to 0-017 min-minus 1 and did not vary significantly from one species to another. 3. Na efflux is rapidly reduced by 30-50% on removing external K or applying ouabain but is reduced by 90% on cooling to 0 degrees C. The effects of K and cooling are also rapidly reversible. 4. Substitution of external Na by Li produces a slow decline of Na efflux. Reversal on restoring external Na is, however, rapid even in the presence of ouabain. 5. When external Na is replaced by Li in the presence of ouabain, the normal decline in Na efflux does not occur. 6. When external Na has been replaced by Li, application of ouabain causes little or no further decline in Na efflux. 7. These results are interpreted quantitatively by means of a model which proposes that intracellular membrane-bounded channels (IMBC) contain 10-30% of the intracellular Na and provide a channel for its expulsion from the cell via connexions with the cell surface. It is supposed that Na is expelled actively from the cytoplasm into the IMBC as well as at the cell surface. Na expulsion via the IMBC is supposed to be insensitive to external K or ouabain. This model accounts for the results using parameters consistent with other investigations by autoradiography and Na-sensitive micro-electrodes. 8. Preliminary electron micrographic evidence shows channels which appear to lead from the cell surface into the cytoplasm and which may correspond with the proposed IMBC of the model.