Stimulation of vascular smooth muscle with bacterial lipopolysaccharide (LPS) and proinflammatory cytokines induces the expression of a distinct isoform of NO synthase (inducible NOS [iNOS]) contributing to the suppression of vascular contractility. We have obtained evidence of the involvement of an indirect pathway triggered by NO and its reaction product peroxynitrite (ONOO⁻) through the activation of the nuclear enzyme poly-ADP ribosyltransferase (PARS) in the pathogenesis of cellular energetic and contractile failure in vascular smooth muscle. Exposure of vascular smooth muscle cells caused DNA strand breaks, activation of PARS, depletion of NAD⁺, and inhibition of mitochondrial respiration. The NAD⁺ depletion and inhibition of mitochondrial respiration were reduced by pharmacological inhibition of PARS. Stimulation of vascular smooth muscle cells with LPS and interferon gamma (IFN-γ) triggered the production of superoxide anion over 3 to 48 hours and NO and ONOO⁻ over 24 to 48 hours and resulted in significant DNA strand breakage. The decrease in mitochondrial respiration in response to LPS and IFN-γ stimulation was inhibited by the ONOO⁻ scavenger uric acid (100 μmol/L) and by inhibitors of iNOS. The PARS inhibitors 3-aminobenzamide (1 mmol/L), nicotinamide (1 mmol/L), and PD 128763 (100 μmol/L) inhibited the reduction in cellular NAD⁺ and ATP and the suppression of mitochondrial respiration in response to LPS and IFN-γ stimulation. Administration of 3-aminobenzamide also reduced PARS activation and vascular hyporeactivity of rat thoracic aortas exposed to ONOO⁻ (300 μmol/L to 1.5 mmol/L) in vitro. 3-Aminobenzamide (10 mg/kg IP) preserved the ex vivo contractility of aortas obtained from endotoxic rats and improved survival in lethal murine endotoxic shock. These data suggest that PARS activation due to iNOS induction (1) is involved in the energetic depletion of vascular smooth muscle cells that express iNOS and (2) contributes to the pathogenesis of vascular energetic and contractile failure in endotoxic shock. Inhibition of PARS may be a novel concept of therapeutic potential in shock.