Maturation of the neuromuscular junction is accompanied by molecular switching of acetylcholine receptor (AChR) channels from embryonic types with gamma-subunits to adult ones with epsilon-subunits after birth. As a step toward understanding the molecular mechanisms of the gamma-to-epsilon switch, we addressed the question of whether embryonic- and adult-type AChRs constitute different endplates during the transitional period. From analyses with double- or triple-staining with anti-gamma- and/or anti-epsilon-antibodies together with alpha-bungarotoxin, which binds to alpha-subunits, we demonstrated that during neonatal stages in mice, adult-type AChRs are incorporated into individual endplates expressing embryonic-AChRs and replace these embryonic-AChRs gradually. The main period of AChR transition in the mouse diaphragm was between postnatal days 5 (P5) and P7, similar to the period described previously in which endplates shift from multi-axon to single-axon innervation. This finding will help our understanding of the mechanisms of the gamma-to-epsilon switch during establishment of the neuromuscular junction.