THE action potentials of vertebrate smooth muscles differ from those of nerve and fast skeletal muscles in several ways. For example, replacement of up to 80 per cent of the sodium salts of the bathing solution with sucrose has little or no effect on the action potential1. Provided that allowance is made for the possible redistribution of Na+ ions in a low Na+ environment and if it is assumed that relatively few channels are available for the inward movement of Na+, it is possible to account for this finding in terms of the ionic theory of Hodgkin and Huxley2. On the other hand, the slow rate of depolarization of the spikes of the taenia coli3, vas deferens4 and uterus5, and the graded nature of the spikes in response to electrical stimulation3,4, are characteristics which resemble the properties of crustacean skeletal muscle. Fatt and Ginsborg6 have shown that the spike mechanism in crustacean muscle involves an increase in conductance for divalent cations rather than Na+ ions. It has been suggested that Ca++ ions may also be involved in the action potentials of smooth muscle7.