Amplitude distributions of miniature endplate potentials (MEPPs) from the isolated mouse diaphragm neuromuscular junctions showed two classes of MEPPs at room temperature. The smaller MEPPs formed a skew distribution which composed about 15% of the MEPPs, the larger MEPPs formed a bell-shaped distribution and represented the classically studied MEPPs. In many MEPP amplitude histograms, both the skew part and bell-shaped part of the histogram showed integral peaks. The first peak is composed of s-MEPPs. The calcium ionophore (X-537A, 10(-5) M) blocked the generation of bell-shaped MEPPs leaving the s-MEPPs. There was little postsynaptic action at this low concentration. Heat challenges reversibly increased the percentage of s-MEPPs and reduced the mean of the bell distribution. Many MEPPs after either challenge had inflections on their rising phases. These observations are evidence that the larger classical MEPP is composed of subunits and the release of one subunit generates the s-MEPP.