1. Membrane potential changes of the smooth muscle cells of the taenia coli were recorded during stimulation of the perivascular inhibitory nerves.2. Some spontaneous action potentials were preceded by a slow pacemaker-like potential. Others began at or near the maximum level of the membrane potential and were not preceded by pacemaker-like potentials.3. There were no changes in the membrane potential of smooth muscle cells when the inhibitory nerves were stimulated with a single pulse. Stimulation at frequencies greater than 5 pulses/sec caused a hyperpolarization of the smooth muscle membrane. This resulted in a decrease in spike frequency and relaxation.4. When the frequency of stimulation of the inhibitory nerves was increased there was an increase in the amplitude and rate of rise of the hyperpolarization and a decrease of the latency. The latency varied from 150 to 300 msec, and the largest hyperpolarization recorded was 16 mV.5. The effect of the hyperpolarization due to nerve stimulation in cells showing pacemaker-like activity was to increase the level of the membrane potential at which the action potentials began and to increase the membrane potential to which the action potentials repolarized. Action potentials which occurred during hyperpolarizations of the membrane had greater rates of rise and fall and larger amplitudes than did the action potentials which occurred before hyperpolarization.6. The effect of the hyperpolarization due to nerve stimulation in cells which did not show pacemaker-like activity depended on the amplitude of the hyperpolarization. Small hyperpolarizations exposed small depolarizations of the membrane which occurred when an action potential would normally have been expected. Large hyperpolarizations blocked the action potentials entirely.7. Action potentials did not begin firing again at the normal rate immediately after stimulation ceased. The time taken for the rate of firing of action potentials to return to normal increased with increasing frequency of stimulation.8. The hyperpolarization in response to perivascular inhibitory nerve stimulation was blocked by guanethidine and bretylium.