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Frog motor nerve terminals were stained with the activity-dependent dye FM1-43, which appears to stain recycled synaptic vesicles. Superficial end plates which could be visualized in their entirety were imaged and end plate potentials (EPPs) evoked by low frequency nerve stimulation were recorded from the muscle fibers which were innervated by the imaged terminals. Curare was present to block muscle contractions. The amplitude of the EPPs correlated reasonably well with the number of fluorescent spots counted in the terminals (r = 0.68). Each fluorescent spot probably represents a cluster of synaptic vesicles localized at an active zone. Several other morphological and electrophysiological values were measured and calculated. The results are consistent with the ideas that FM1-43 stains recycled synaptic vesicles, and that the number of vesicle clusters in a terminal is a good predictor of synaptic efficacy.