Nitric oxide (NO) production through the inducible nitric-oxide synthase (iNOS) pathway is increased in inflammatory diseases and leads to cellular injury. Anti-inflammatory steroids inhibit the expression of various inflammatory genes, including iNOS. In the present study, we investigated the mechanism how dexamethasone decreased NO production in murine J774 macrophages. Dexamethasone (0.1-10 microM) inhibited the production of NO and iNOS protein in a dose-dependent manner in cells stimulated with lipopolysaccharides (LPS). In contrast, in cells treated with a combination of LPS and interferon-gamma (IFN-gamma), dexamethasone did not reduce iNOS expression and NO formation. Dissociated glucocorticoid RU24858 inhibited iNOS expression and NO production to levels comparable with that of dexamethasone, suggesting that the reduced iNOS expression by dexamethasone is not a GRE-mediated event. In further studies, the effect of dexamethasone on iNOS mRNA levels was tested by actinomycin assay. The half-life of iNOS mRNA after LPS treatment was 5 h 40 min, and dexamethasone reduced it to 3 h. The increased degradation of iNOS mRNA was reversed by a protein synthesis inhibitor cycloheximide. iNOS mRNA was more stabile in cells treated with a combination of LPS plus IFN-gamma (half-life = 8 h 20 min), and dexamethasone had a minor effect in these conditions. In conclusion, dexamethasone decreases iNOS-dependent NO production by destabilizing iNOS mRNA in LPS-treated cells by a mechanism that requires de novo protein synthesis. Also, decreased iNOS mRNA and protein expression and NO formation by dexamethasone was not found in cells treated with a combination of LPS plus IFN-gamma, suggesting that the effect of dexamethasone is stimulus-dependent.