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1. Membrane potentials and tension were recorded during nerve stimulation and direct stimulation of smooth muscle cells of the anterior byssus retractor muscle of Mytilus edulis L.

2. The resting potential averaged 65 mV (range 55-72 mV).

3. Junction potentials reached 25 mV and decayed to one half maximum amplitude in 500 msec. Spatial summation and facilitation of junction potentials were observed.

4. Action potentials, 50 msec in duration and up to 50 mV in amplitude were fired at a membrane potential of 35-40 mV. No overshoot was observed.

5. Contraction in response to neural stimulation was associated with spike discharge. Measurement of tension and depolarization in muscle bundles at high K⁺ indicated that tension is only produced at membrane potentials similar to those achieved by spike discharge.

6. Blocking of junction potentials, spike discharge and contraction by methantheline, an acetylcholine antagonist, supports the hypothesis that the muscle is excited by cholinergic nerves. However, evidence of a presynaptic action of methantheline complicates this argument.