1. The presynaptic mechanisms governing the release and recycling of synaptic vesicles have been studied by examining the effects of nerve stimulation, (-)-vesamicol (an inhibitor of acetylcholine transport into synaptic vesicles) and troxypyrrolium (an inhibitor of the high-affinity, sodium-dependent, choline uptake system) on endplate currents (EPCs) and miniature endplate currents (MECPs) recorded from motor endplates in cut rat hemidiaphragm preparations. 2. In control experiments, 5 min of 10 Hz nerve stimulation had no effect on either the mean or the distribution of MEPC amplitudes. 3. Nerve stimulation in the presence of (-)-vesamicol (25 nM-10 microM) revealed a population of MEPCs that was unaffected by the compound and a population of MEPCs whose mean amplitude was selectively reduced by the compound. 4. Nerve stimulation in the presence of troxypyrrolium (20 microM) produced a uniform reduction in the amplitude of all MEPCs with no change in the coefficient of variance of MEPC amplitudes. 5. The concentration-dependent effects of (-)-vesamicol on the amplitude of the evoked EPCs paralleled the concentration-dependent effects of the compound on MEPC amplitudes. 6. The results are consistent with the hypothesis that both recycled and performed synaptic vesicles are heterogeneously released from rat motor nerve terminals and that (-)-vesamicol acts selectively on recycling vesicles. In addition, a model of vascular loading that accounts for the different effects of nerve stimulation on MEPC amplitudes in the presence of (-)-vesamicol and troxypyrrolium is described.