The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, statins, are potent inhibitors of cholesterol synthesis and have wide therapeutic use in cardiovascular diseases. Recent evidence, however, suggests that the beneficial effects of statins may extend beyond their action on serum cholesterol levels. In this study, we investigated the effects of lovastatin, pravastatin, atorvastatin and fluvastatin on macrophage formation of nitric oxide (NO) in murine RAW 264.7 cells. Stimulation of macrophages with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) resulted in inducible NO synthase (iNOS) expression, which was accompanied by a large amount of NO formation. At concentrations of 0.1-30 microM, statins can inhibit stimuli-induced NO formation and iNOS induction to different extents. This inhibition occurs at the transcriptional level, and displays potency in the order of lovastatin > atorvastatin > fluvastatin >> pravastatin. We found that LPS-induced I kappa B kinase and nuclear factor-kappa B (NF-kappa B) activation, as well as IFN-gamma-induced signal transducer and activator of transcription 1 (STAT1) phosphorylation, were reduced by lovastatin. Moreover, inhibition by lovastatin of NO production and kappa B activation was reversed by mevalonate, geranylgeranyl pyrophosphate and farnesyl pyrophosphate. All these results suggest that inhibition of iNOS gene expression by statins can be attributed to interference with protein isoprenylation, which mediates both NF-kappa B and STAT1 activation in the upstream signaling pathways for iNOS gene transcription.