Controlled mechanical ventilation (CMV) results in diaphragmatic dysfunction. Oxidative stress is an important contributor to ventilator-induced diaphragm dysfunction, since 18 hours of CMV lead to increased protein oxidation and increased lipid peroxidation. We hypothesized that administration of an antioxidant, N-acetylcysteine (NAC), would restore the redox balance in the diaphragm and prevent the deleterious effects of CMV.