Skeletal muscle precursor cells are adult stem cells located among muscle fibers. Proliferation, migration, and subsequent differentiation of these cells are critical steps in the repair of muscle injury. We document in this study the roles and mechanisms through which the NAPDH oxidase complex regulates muscle precursor cell proliferation. The NADPH oxidase subunits Nox2, Nox4, p22(phox), p47(phox), and p67(phox) were detected in primary human and murine skeletal muscle precursor cells. In human muscle precursor cells, NADPH oxidase-fusion proteins were localized in the cytosolic and membrane compartments of the cell, except for p47(phox), which was detected in the nucleus. In proliferating subconfluent precursor cells, both Nox2 and Nox4 contributed to O(2)(-) production. However, Nox4 expression was significantly attenuated in differentiated myotubes. Proliferation of precursor cells was significantly reduced by antioxidants (N-acetylcysteine and apocynin), inhibition of p22(phox) expression by using siRNA oligonucleotides, and reduction of Nox4 and p47(phox) activities with dominant-negative vectors and siRNA oligonucleotides resulted in attenuation of activities of the Erk1/2, PI-3 kinase/AKT and NFkappaB pathways and significant reduction in cyclin D1 levels. We conclude that NADPH oxidase is expressed in skeletal muscle precursor cells and that its activity plays an important role in promoting proliferation of these cells.