We showed that a LacZ expression plasmid (pCAG-lacZ) injection followed by electroporation increased the expression of the LacZ gene in the skeletal muscles of adult mdx mice up to ninefold higher as compared with simple intramuscular DNA injection. When full-length mouse dystrophin plasmid (pCAG-dys) and pCAG-lacZ were co-transfected by electroporation, 56% of dystrophin-positive fibers were stained for beta-galactosidase activity suggesting most of these myofibers are not revertants but transfected ones. Our data indicate that electroporation in vivo could introduce large full-length dystrophin cDNA into skeletal muscle of adult mdx mice.