Amplitude histograms of spontaneous miniature endplate potentials (MEPPs) from adult sartorius muscle cells show a definite bimodality with the mean amplitude of the larger mode five to seven times that of the smaller mode which accounted for 2-5 % of the total MEPPs. Histograms were plotted after high frequency MEPP generation induced by increasing temperature, increasing external calcium or nerve stimulation. These plots showed a reversible left-shift of the major mode as well as a reversible increase in the proportion of small mode MEPPs. Repeated challenges shifted almost all MEPPs into the small mode. An increase in the percentage of small mode MEPPs also occurred spontaneously during the course of denervation before the quiescent period and some of the histogram profiles showed multiple modes whose means were integer multiples of the small mode mean. In the early stages of hind leg development the greatest proportion of MEPPs were of the small mode size; as metamorphosis progressed, the histograms showed a definite multimodality with the mean of each mode being an integer multiple of the small mode mean and with the proportion of MEPPs in each mode about the same. During tail resorption the percentage of larger MEPPs increased until the adult histogram profile was reached. Thus, the changes in MEPP amplitude histograms over the course of metamorphosis are the reverse of those found with denervation.