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1. The intracellular concentration and transmembrane flux of Na in smooth muscle cells of sheep carotid arteries were measured by identifying a fraction of tissue Na whose efflux was markedly sensitive to removal of external Ca. This allowed intracellular Na to be distinguished from extracellular bound Na exchanging with a similar time constant.

2. When the arteries were in solution containing Na 148 mM and Ca 2·5 mM the mean intracellular Na concentration was 7·3 m-mole/kg cell water and the mean transmembrane flux of Na was 0·18 p-mole cm^-2 sec^-1, both values being much lower than reported values for intestinal smooth muscle.

3. During electrical activity induced by Ca deprivation the influx of Na increased to 3·2 p-mole cm^-2 sec^-1; a 50% reduction in Na concentration stopped electrical activity and reduced influx by more than 50%, and the excess Na influx per action potential was calculated to be at least 1·0-1·5 p-mole cm^-2.

4. Arteries in physiological saline (Ca 2·5 mM) contained 3·48 µmole Ca/g, much of which was bound extracellularly; in Ca-free saline 1·39 µmole Ca/g left the tissue within 3 min, showing that extracellular dissolved Ca diffused freely out of the tissue.

5. The results provide further evidence that Na is the principal ion carrying the depolarizing current of action potentials in this smooth muscle.