The frequency distribution of miniature endplate potential (mepp) amplitudes was examined in Xenopus nerve-muscle cultures. In the early days of nerve-muscle co-culture (1-2 days) the mepp amplitude histogram was skewed in the majority of muscle cells and had a peak approximately at 0.6 mV. Innervated muscle cells in older cultures (3-4 days), however, often had two peaks. The distribution at the smaller amplitude was skewed and the peak was approximately at 0.6 mV, whereas the peak at the larger amplitude was more-or-less symmetrical and about 18 times greater than the small one. The time-to-peak value of mepps in the peak at the smaller amplitude was slightly smaller than those in the peak at the larger amplitude. Small mepps were not due to electrotonic spread of mepps generated at distant sites nor due to focal development of acetylcholinesterase. Two peaks were also observed in the amplitude histogram of extracellularly recorded miniature endplate currents. Therefore, these two types of mepps must be generated practically at the same site. Mepps in the peak at the smaller amplitude had similar properties as sub-miniature endplate potentials (s-mepps) described in the adult animal. The second symmetrical peak in older cultures may correspond to classical mepps (c-mepps). The age-dependent shift from the amplitude distribution with one peak to that with two peaks has also been reported in developing and regenerating neuromuscular junctions. Therefore, I suggest that during development in culture, mepps appear at first as s-mepps and subsequently c-mepps develop, forming the second peak in the amplitude histogram.