Altered nitric oxide (NO) production/release is involved in gastrointestinal motor disorders occurring in dystrophic (mdx) mice. Since the hormone relaxin (RLX) can up-regulate NO biosynthesis, its effects on spontaneous motility and NO synthase (NOS) expression in the ileum of dystrophic (mdx) mice were investigated. Mechanical responses of ileal preparations were recorded in vitro via force-displacement transducers. Evaluation of the expression of NOS isoforms was performed by immunohistochemistry and Western blot. Normal and mdx mice were distributed into three groups: untreated, RLX-pretreated, vehicle-pretreated. Ileal preparations from the untreated animals showed spontaneous muscular contractions whose amplitude was significantly higher in mdx than in normal mice. Addition of RLX, alone or together with L-arginine, to the bath medium depressed the amplitude of the contractions in the mdx mice, thus re-establishing a motility pattern typical of the normal mice. The NOS inhibitor L-NNA or the guanylate cyclase inhibitor ODQ reversed the effects of RLX. In RLX-pretreated mdx mice, the amplitude of spontaneous motility was reduced, thus resembling that of the normal mice, and NOS II expression in the muscle coat was increased in respect to the vehicle-pretreated mdx animals. These results indicate that RLX can reverse the altered ileal motility of mdx mice to a normal pattern, likely by up-regulating NOS II expression and NO biosynthesis in the ileal smooth muscle.