NO is a crucial mediator of the inflammatory response, but its in vivo role as a determinant of lung inflammation remains unclear. We addressed the in vivo role of NO in regulating the activation of NF-kappaB and expression of inflammatory proteins using an in vivo mouse model of sepsis induced by i.p. injection of Escherichia coli. We observed time-dependent degradation of IkappaB and activation of NF-kappaB accompanied by increases in inducible NOS, macrophage inflammatory protein-2 (MIP-2), and ICAM-1 expression after E. coli challenge, which paralleled the ability of lung tissue to produce high-output NO. To determine the role of NO in this process, mice were pretreated with the NO synthase (NOS) inhibitor NG-methyl-L-arginine. Despite having relatively modest effects on NF-kappaB activation and ICAM-1 or inducible NOS expression, the NOS inhibitor almost completely inhibited expression of MIP-2 in response to E. coli challenge. These responses were associated with the inhibition of migration of neutrophils in lung tissue and increased permeability induced by E. coli. In mice pretreated with NG-methyl-L-arginine, coadministration of E. coli with the NO donor (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate substantially restored MIP-2 expression but decreased ICAM-1 expression. The results suggest that NO generated after administration of E. coli serves as an important proinflammatory signal to up-regulate MIP-2 expression in vivo. Thus, NO production in high quantities may be important in the mechanism of amplification of the lung inflammatory response associated with sepsis.