Experiments were done to study the relationship between miniature endplate potential (MEPP) frequency and MEPP amplitude in different regions along the frog neuromuscular junction (NMJ). The position of the release site producing a MEPP and the amplitude of a MEPP at its release site were evaluated by using the spatial decay method after simultaneous intracellular recordings at each distal end of the NMJ. The NMJ was divided in 10 regions of equal length. It was found that MEPP amplitude and MEPP frequency are smaller in distal regions than in proximal ones. Moreover, we observed a logarithmic relationship between MEPP frequency of a given region and the mean MEPP amplitude of the same region. These results suggest that the probability to produce a MEPP and the quantity of neurotransmitter liberated are two synaptic functions controlled or affected by common mechanisms.