1. Single sucrose gap recordings showed that spontaneous action potentials of rat ileal smooth muscle consisted of slow waves and superimposed spikes which generated rhythmic contractions. As external potassium was raised, the resting potential progressively depolarized.

2. Calcium-free salines inhibited spontaneous mechanical activity and inhibited the plateau phase of the action potential, but spontaneous spike depolarizations persisted.

3. Verapamil, nifedipine and diltiazem all inhibited spontaneous mechanical activity and the plateau phase of the action potential, while in addition diltiazem augmented spike amplitude.

4. Mn ions also inhibited mechanical activity and the action potential plateau, without affecting spike activity while the calcium ionophore A23187 enhanced both mechanical and electrical activity with a pronounced effect on spike amplitude.

5. These results are consistent with the view that the plateau phase of the ileal smooth muscle action potential is dependent upon an influx of extracellular calcium possibly through voltage dependent slow calcium channels.