Rationale Previous workers have demonstrated that controlled mechanical ventilation (CMV) results in diaphragm inactivity and elicits a rapid development of diaphragm weakness due to both contractile dysfunction and fiber atrophy. Limited data exist regarding the impact of pressure support ventilation (PSV),a commonly used mode of mechanical ventilation--that permits partial mechanical activity of the diaphragm—on diaphragm structure and function. Objectives We carried out the present study to test the hypothesis that high level PSV decreases the diaphragm pathology associated with CMV. Methods Sprague-Dawley rats were randomly assigned to one of the following five groups:1) control (no mechanical ventilation); 2) 12 hours of CMV (12CMV); 3) 18 hours of CMV (18CMV); 4) 12 hours of PSV (12PSV); or 5) 18 hours of PSV (18PSV). Measurements and main results We carried out the following measurements on diaphragm specimens: 4-hydroxynonenal (HNE)—a marker of oxidative stress, active caspase-3 (casp-3),active calpain-1 (calp-1), fiber type cross-sectional area (CSA), and specific force (sp F). Compared to control, both 12PSV and 18PSV promoted a significant decrement in diaphragmatic specific force production, but to a lesser degree than 12CMV and 18CMV. Further, 12CMV, 18PSV, and 18CMV resulted in significant atrophy in all diaphragm fiber types, as well as significant increases in a biomarker of oxidative stress (4-HNE) and increased proteolytic activity (20S proteasome, calpain-1, and caspase-3). Further, while no inspiratory effort occurs during CMV, it was observed that PSV resulted in large decrement, ~96%, in inspiratory effort compared to spontaneous breathing animals. Conclusion High levels of prolonged PSV promotes diaphragmatic atrophy and contractile dysfunction. Further, similar to CMV, PSV-induced diaphragmatic atrophy and weakness is associated with both diaphragmatic oxidative stress and protease activation. .