Triggering of the macrophage cell line RAW 264.7 with LPS promotes a transient activation of phosphatidylinositol 3-kinase (PI3-kinase). Incubation of activated macrophages with wortmannin and LY294002, two inhibitors of PI3-kinase, increased the amount of inducible nitric oxide synthase (iNOS) and the synthesis of nitric oxide. Treatment with wortmannin promoted a prolonged activation of NF-kappaB in LPS-treated cells as well as an increase in the promoter activity of the iNOS gene as deduced from transfection experiments using a 1.7-kb fragment of the 5' flanking region of the iNOS gene. Cotransfection of cells with a catalytically active p110 subunit of PI3-kinase impaired the responsiveness of the iNOS promoter to LPS stimulation, whereas transfection with a kinase-deficient mutant of p110 maintained the up-regulation in response to wortmannin. These results indicate that PI3-kinase plays a negative role in the process of macrophage activation and suggest that this enzyme might participate in the mechanism of action of antiinflammatory cytokines.