Pentobarbital-anesthetized rats subjected to traumatic shock developed a shock state characterized by marked hypotension to 65-70 mmHg, a survival time of 88 ± 13 min, significant increases in ileal myeloperoxidase activity (P < 0.01), and severe endothelial dysfunction as evidenced by a significant (P < 0.01) decrease in vasorelaxation to endothelium-dependent dilators. Treatment with heparinase III (45 µg · kg¹ · min¹) 10 min posttrauma prolonged survival time to 223 ± 19 min (P < 0.001), significantly attenuated ileal myeloperoxidase activity (P < 0.01), and significantly preserved endothelial function (P < 0.05). Intravital microscopy of the rat mesentery showed that infusion of heparinase III (45-67 µg · kg¹ · min¹) significantly (P < 0.01) attenuated both leukocyte rolling and adherence in the rat mesenteric microvasculature in response to N^G-nitro-L-arginine methyl ester stimulation. Immunohistochemical localization of surface-expressed P-selectin on mesenteric venules showed that heparinase III infusion at 45-67 µg · kg¹ · min¹ significantly (P < 0.05) attenuated the increase in surface P-selectin expression. The beneficial effects of heparinase III are mediated at least in part by attenuating leukocyte-endothelial cell interactions via a P-selectin-dependent mechanism.