During systemic inflammation recruitment and activation of leukocytes in the pulmonary microcirculation may result in a potentially life threatening acute lung injury. We elucidated the role of the poly (ADP-ribose) synthetase (PARS), a nucleotide-polymerizising enzyme, in regulation of leukocyte recruitment within the lung with regard to the localization in the pulmonary microcirculation and in correlation to hemodynamics in the respective vascular segments and expression of the intercellular adhesion molecule 1 during endotoxemia. Inhibition of PARS by 3-aminobenzamide reduced the endotoxin-induced leukocyte recruitment within pulmonary arterioles, capillaries, and venules in rabbits as quantified by in vivo fluorescence microscopy. Microhemodynamics and thus shear rates in all pulmonary microvascular segments remained constant. Simultaneously, inhibition of PARS with 3-aminobenzamide suppressed the endotoxin-induced adhesion molecules expression as demonstrated for the intercellular adhesion molecule 1 by immunohistochemistry and Western blot analysis. This result was confirmed by PARS knock-out mice. The inhibitory effect of 3-aminobenzamide on leukocyte recruitment was associated with a reduction of pulmonary capillary leakage and edema formation. We first provide evidence that PARS activation mediates the leukocyte sequestration in pulmonary microvessels through up-regulation of adhesion molecules. As reactive oxygen species released from leukocyte are supposed to cause an up-regulation of adhesion molecules we conclude that PARS inhibition contributes to termination of this vicious circle and inhibits the inflammatory process.