1. An analysis has been made of the transmission of excitation from the hypogastric nerve to the smooth muscle cells of the guinea-pig vas deferens.2. Depolarization of single muscle cells with current pulses from an intracellular electrode gave local depolarizations of the cell membrane which were not propagated. The total membrane resistance after 100 msec of depolarization was 15 MOmega for depolarizations between 10 and 40 mV.3. Depolarization of some cell membranes with a current pulse during the excitatory junction potential (E.J.P.) decreased the amplitude of the E.J.P. from about 10 mV at 20 mV depolarization, to nearly zero at 60 mV depolarization. In some cells the E.J.P. was unchanged during depolarizations of 50 mV.4. The action of transmitter on the smooth muscle cell membrane continued for the duration of the E.J.P. Action potentials which occurred at various times during the E.J.P. failed to remove the remaining phases of the E.J.P.5. It was shown that the slow time course of the E.J.P. could not be due to the instantaneous and simultaneous release of transmitter from a number of relatively distant sources.6. It was shown that each smooth muscle cell was innervated by several axons. The serial sections examined with the electron microscope showed that a smooth muscle had either a single axon terminating within 200 A of the muscle or no axons terminating on it at all. Therefore transmitter must be released along the length of the axons as well as at the terminations of the axons.