To elucidate the role of phosphatidylinositol-3-kinase (PI3K) during macrophage activation, we examined the effects of wortmannin, a specific inhibitor of PI3K, on the induction of nitric oxide (NO) synthesis and tumor necrosis factor-alpha (TNF-alpha) secretion from lipopolysaccharide (LPS)-stimulated macrophages. Wortmannin had no effects on NO synthesis and TNF-alpha secretion by itself. Wortmannin markedly potentiated the LPS-induced NO production in a dose-dependent manner. Western blot analysis demonstrated that significantly increased levels of iNOS protein were expressed in LPS-stimulated macrophages treated with wortmannin, compared to those without LPS. Furthermore, enhancement of TNF-alpha secretion was observed in the initiation stage for activation of LPS-stimulated macrophages treated with wortmannin. These results suggest that PI3K plays an important role in transducing the signal that is involved in LPS-induced macrophage activation.