Duchenne muscular dystrophy (DMD) is characterized by skeletal muscle weakness that extends to the respiratory musculature. In DMD, the diaphragm muscle is highly susceptible to inflammation and oxidative stress and death ultimately occurs due to cardio-respiratory failure. Oxidative stress occurs due to increased reactive oxygen species (ROS) and/or decreased antioxidant capacity. Oxidative stress has been described in the respiratory and locomotor muscles of DMD patients. NADPH oxidase (NOX) is a ROS-generating complex and a putative candidate for mediating ROS production in DMD. We hypothesized that NOX contributes to respiratory muscle weakness in dystrophin deficient mdx mice.   Diaphragm muscle isometric and isotonic performance were determined ex vivo for 2- and 6-month old wild-type (n=19) and mdx (n=19) mice. We used apocynin (putative NOX inhibitor), to assess the contribution of NOX to diaphragm muscle weakness in mdx mice. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to determine the mRNA expression of NOX isoforms (NOX1, NOX2, NOX4, Duox1, Duox2) and NOX regulatory subunits (p22phox, p40phox, p47phox, p67phox, Rac) in the diaphragm muscle of both 2- and 6-month-old wild-type and mdx mice. Data were statistically compared using two-way ANOVA with Bonferroni post hoc test. P<0.05 was considered statistically significant.   Profound diaphragm muscle weakness was observed in 2- and 6-month-old mdx mice in comparison to wild-type. Apocynin had a positive inotropic effect in both wild-type and mdx mice at 2 months of age, increasing force-generating capacity. In the 6-month-old mice, apocynin did not improve force-generating capacity. The mRNA expression of NOX isoforms (NOX1, NOX2, NOX4, Duox1) were significantly increased in the diaphragm muscle of mdx mice compared to wild-type at both ages. Similarly, diaphragm mRNA expression of the NOX regulatory subunits (p22phox, p40phox, p47phox, p67phox, Rac) was significantly increased for mdx compared to wild-type. Diaphragm mRNA expression of the NOX regulatory subunit p40phox was significantly decreased for wild-type diaphragm at six- compared to two-months of age. Diaphragm mRNA expression of the NOX isoform NOX2 and NOX regulatory subunits (p22phox, p40phox, p47phox, p67phox, Rac) were significantly decreased for mdx mice at six- compared to two-months of age.   Functional experiments using apocynin suggest an age-dependent decline in the physiological role of NOX in the diaphragm. These studies also demonstrate that there is an age-related decline in the mRNA expression of NOX and associated isoforms in mdx diaphragm. NOX mRNA expression is increased in the diaphragm muscle of mdx mice compared to wild-type. Apocynin increased mdx diaphragm force. These data extend our understanding of redox remodelling in dystrophic respiratory muscle and are relevant to the search for new therapeutic targets. Physiological suppression of NOX may have application in the treatment of muscular dystrophy in early disease.