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1. Strips of smooth muscle from common carotid arteries of sheep were electrically quiescent in solution containing Na 148 mM and Ca 2·5 mM.

2. When Ca was removed they became electrically active. Addition of low concentrations of Ca (0·025-0·075 mM) or Mg (0·025-0·750 mM) stopped their activity while ethylenediamine tetra-acetate (EDTA) (1·25 mM) accelerated it.

3. Replacement of Na by Tris or choline stopped the activity in Ca-free solution. After partial replacement of Na electrical activity could be restored by lowering the resting potential but after complete replacement of Na it could not.

4. In the presence of Ca (2·5 mM) small spikes could sometimes be induced after 20 min in Na-free Tris solution by lowering the resting potential by an increase in the external K concentration.

5. The results indicate that the depolarizing current of action potentials in this smooth muscle was largely carried by Na, although a little may have been carried by Ca in Ca-containing solutions.

6. The arteries in general resembled striated muscle rather than intestinal smooth muscle in these respects, but unlike striated muscle their action potentials were not stopped by tetrodotoxin.

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