Rationale: The mechanisms by which infections induce diaphragm dysfunction remain poorly understood. The purpose of this study was to determine which caspase pathways (i.e. the extrinsic, death receptor linked caspase 8 pathway and/or the intrinsic, mitochondrial related caspase 9 pathway) are responsible for endotoxin induced diaphragm contractile dysfunction. Methods: We determined: (a) if endotoxin administration (12 mg/kg, intraperitoneally) to mice induces caspase 8 or 9 activation in the diaphragm, (b) if administration of a caspase 8 inhibitor (Ac-IETD-CHO, N-acetyl-Ile-Glu-Thr-Asp-CHO, 3 mg/kg IV) or a caspase 9 inhibitor (Ac-LEHD-CHO, N-acetyl-Leu-Glu-His-Asp-CHO, 3 mg/kg IV) blocks endotoxin induced diaphragmatic weakness and caspase 3 activation, (c) if Tumor Necrosis Factor Receptor 1 (TNFR1) deficient mice have reduced caspase activation and diaphragm dysfunction following endotoxin, and (d) if cytokines [Tumor Necrosis Factor- (TNF) or cytomix, a mixture of TNF, Interleukin-1, Interferon-, and endotoxin] evoke caspase activation in C2C12 myotubes. Results: Endotoxin markedly reduced diaphragm force generation (p<0.001) and induced increases in caspase 3 and caspase 8 activity (p<0.03) but failed to increase caspase 9. Inhibitors of caspase 8, but not of caspase 9, prevented endotoxin induced reductions in diaphragm force and caspase 3 activation (p<0.01). Mice deficient in TNFR1 also had reduced caspase 8 activation (p<0.001) and less contractile dysfunction (p<0.01) after endotoxin. Furthermore, incubation of C2C12 cells with either TNF or cytomix elicited significant caspase 8 activation. Conclusions: The caspase 8 pathway is strongly activated in the diaphragm following endotoxin and is responsible for caspase 3 activation and diaphragm weakness.