Background and purpose: Nitric oxide (NO) production through the inducible nitric oxide synthase (iNOS) pathway is increased in response to pro-inflammatory cytokines and bacterial products. In inflammation, NO has pro-inflammatory and regulatory effects. Peroxisome proliferator-activated receptors (PPARs), members of the nuclear steroid receptor superfamily, regulate not only metabolic but also inflammatory processes. The aim of the present study was to investigate the role of PPARa in the regulation of NO production and iNOS expression in activated macrophages. Experimental approach: The effects of PPARa agonists were investigated on iNOS mRNA and protein expression, on NO production and on the activation of transcription factors NF-kB and STAT1 in J774 murine macrophages exposed to bacterial lipopolysaccharide (LPS). Key results: PPARa agonists GW7647 and WY14643 reduced LPS-induced NO production in a dose-dependent manner as measured by the accumulation of nitrite into the culture medium. However, PPARa agonists did not alter LPS-induced iNOS mRNA expression or activation of NF-kB or STAT1 which are important transcription factors for iNOS. Nevertheless, iNOS protein levels were reduced by PPARa agonists in a time-dependent manner. The reduction was markedly greater after 24 h incubation than after 8 h incubation. Treatment with the proteasome inhibitors, lactacystin or MG132, reversed the decrease in iNOS protein levels caused by PPARa agonists. Conclusions and implications:The results suggest that PPARa agonists reduce LPS-induced iNOS expression and NO production in macrophages by enhancing iNOS protein degradation through the proteasome pathway. The results offer an additional mechanism underlying the anti-inflammatory effects of PPARa agonists.