1. The effects of polarizing current on single smooth muscle cells of the isolated guinea-pig vas deferens have been studied by means of a bridge circuit in which the same electrode was used to pass current and record membrane potential.2. Most cells had polarization resistances from 15 to 37 MΩ.3. Depolarizing currents caused graded spikes up to a maximum amplitude of 75 mV.4. Self-regenerative spikes occurred in some cells if the membrane was depolarized by > 35 mV.5. Currents of long duration and sufficient intensity caused a repetitive discharge.6. Spikes arising from excitatory junction potentials were always `all-or-none' in contrast to the graded responses to intracellular stimulation.7. Electrotonic potentials declined much more rapidly than spontaneous excitatory junction potentials (EJPs); thus the time course of the EJP cannot be determined by the passive electrical properties of the smooth muscle cell membrane.
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