1. Excitatory junction potentials (EJPs) recorded in isolated segments of the proximal main ventral artery of the rat tail were reduced in amplitude and prolonged in time course as temperature was lowered from 35 to 15 degrees C. 2. The slow depolarization that followed the EJPs after supramaximal or repetitive perivascular stimulation was markedly slowed in time course, but little affected in amplitude, as temperature was lowered. 3. The time constant (tau EJP) of the exponential decay phase of the EJP recorded from cells deep in the media was similar to the membrane time constant, so that the increase in tau EJP at low temperatures is consistent with a decrease in membrane conductance. 4. The value of tau EJP was also prolonged if the EJP was evoked at the time of the peak of the slow depolarization; this effect was blocked by idazoxan (10(-7) M) but not by prazosin (10(-6) M). 5. During repeated short bursts of high-frequency stimulation, action potential initiation was facilitated by both the prolongation of EJPs and summation of slow depolarizations; these effects were greater at 25 than at 35 degrees C. 6. The interactions between EJPs and alpha-adrenoreceptor-mediated membrane conductance changes are considered with respect to the electrical events occurring during sympathetic neuromuscular transmission at the natural temperatures of the rat tail.