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External Na has been shown to initiate a loss of 24 Na from high-Na smooth muscle of the guinea-pig taenia coli. This is an ouabain-insensitive effect, and there appears to be a 1:1 exchange of Na ions, suggesting a classical Na/Na exchange mechanism. 2. The Na/Na exchange has many properties in common with a similar exchange studied in high-Na beef erythrocytes by Motais (1973), and Motais & Sola (1973). It is very temperature-sensitive, it is partially inhibited by the sulphydryl reagents studied and it has a fairly low external affinity for Na. 3. The affinity of the external site for alkali metal cations is Na greater than Li greater than K greater than Rb greater than Cs. 4. It has proved impossible to estimate the affinity of the intracellular sites for Na. The curve relating intracellular Na content with the stimulated efflux reaches a maximum and then declines slightly. 5. Another unexpected finding was that after the rate of loss of Na has been reduced in a Na-free medium, reintroducing Na causes an overshoot in the rate, it increases to a value beyond the original one, and then slowly declines to it. 6. Unlike the classical Na/Na mechanism, the process is reduced, but not abolished by metabolic inhibition that depletes the tissue of ATP. 7. The results are interpreted to suggest that the Na/Na exchange is occurring from a cellular compartment of limited volume, which is itself exchanging with the main cell compartment. It is suggested that this small compartment is the sarcoplasmic reticulum, and the effect of metabolic inhibition is to interfere in some way with the relationship between this compartment and the cell membrane.