1. Electrotonic potentials were recorded from the superficial smooth muscle cells of the guinea-pig vas deferens using the method of Abe & Tomita (1968), in response to low-amplitude, long-duration (greater than or equal to 2 sec) pulses. 2. Averaging techniques were used to increase the signal/noise ratio, and the intracellularly recorded electrotonic potentials were corrected for extracellular voltage drop across the bath series resistance. 3. Since the length of tissue in the stimulating and recording compartments affects the time course of electrotonic potentials (see Appendix and Bywater & Redman, 1978) the passive membrane properties were measured with known amounts of tissue in these two compartments. 4. The length constant (lambda) was 0.86 mm and the membrane time constant (tau m) 270 msec. 5. Excitatory junction potentials (e.j.p.s) were recorded and averaged in response to field stimulation of intact branches of the hypogastric nerve. The mean time constant of the exponential decay phase of the e.j.p. (288 msec) was similar to the membrane time constant (tau m = 270 msec). 6. As the e.j.p.s showed little change in amplitude or time constant of decay when recorded up to several millimetres from the stimulating electrode it was assumed that the tissue was isopotential during the e.j.p., and an estimate was made of the time course of the underlying junctional current. 7. The estimated time course of the junctional current during an e.j.p. was similar to the observed time course of a spontaneous junction potential (s.e.j.p.). 8. As the time course of the junctional current during an s.e.j.p.is similar to the time course of the potential change it is likely that the factors which determine the time current underlying the s.e.j.p. also determine the time course of the e.j.p. current.