The septic shock-induced decrease in mesenteric blood flow and release of proinflammatory cytokines are among the major pathophysiologic changes presumed to lead to multiple organ dysfunction syndrome (MODS). Increased nitric oxide (NO) levels are associated with both decreased mesenteric blood flow and positive modulation of proinflammatory cytokine release. In this study we aimed to determine the effect of the timing of the inhibition of nitric oxide synthase (NOS) on mesenteric blood flow and serum interleukin-10 (IL-10) concentrations during endotoxin shock. A nonspecific NOS inhibitor N^G-nitro-l-arginine methyl ester (l-NAME), a specific NOS inhibitor aminoguanidine (AG), or placebo were injected 20 minutes before or 20 minutes after a lipopolysaccharide (LPS) or placebo challenge to Swiss-albino mice, as pretreatment or posttreatment, respectively. At 120 minutes after LPS or placebo injection the mesenteric blood flow was measured, and blood samples from the heart were obtained for IL-10 levels in both groups. Pretreatment and posttreatment with both NOS inhibitors prevented the LPS-induced decrease in mesenteric blood flow. Pretreatment was more effective for this purpose. Pretreatment accentuated the LPS-induced increase in serum IL-10 concentrations, whereas posttreatment had no significant effect. We conclude that the timing of NOS inhibition is important for attenuating some deleterious effects of endotoxin.