Amyloid precursor protein (APP), associated with Alzheimer's disease plaques, is known to be present in synapses of the brain and in the adult neuromuscular junction (NMJ). In the present study we examined protein and gene expression of APP during the development of mouse skeletal muscle. Using immunocytochemical approaches, we found that APP is first detected in myotube cytoplasm at embryonic day 16 and becomes progressively concentrated at the NMJ beginning at birth until adulthood. The colocalization between APP and acetylcholine receptors at the NMJ is only partial at birth, but becomes complete upon reaching adulthood. We observed that all APP isoforms, including the Kunitz-containing (protease inhibitor or KPI) forms, are up-regulated from birth to postnatal day 5 and then decreased to reach the low levels observed in the adult. This suggests the involvement of APP during the events which lead to a mature mono-innervated synapse. A 92-kDa band, characteristic of a cleaved APP695 isoform and not due to a new muscle-specific alternative spliced form, was observed from postnatal day 15 following completion of polyneuronal synapse elimination. Taken together, these data suggest that skeletal muscle APP may well play a role in the differentiation of skeletal muscle and in the formation and maturation of NMJs.