In vitro and in vivo studies have demonstrated that methylguanidine, an inhibitor of nitric oxide synthase (NOS), is also able to reduce tumour necrosis factor-alpha (TNF-alpha) release. In the present study, we evaluated the anti-inflammatory potential of methylguanidine treatment in two models of acute inflammation (carrageenan-induced paw edema and pleurisy) where oxyradical, nitric oxide (NO) and prostaglandins play a crucial role in the inflammatory processes. Our data show that methylguanidine, given intraperitoneally at the dose of 30 mg/kg, inhibits the inflammatory response reducing significantly (P<0.05) paw swelling, pleural exudates formation, mononuclear cell infiltration and histological injury. Furthermore, our data suggests that there is a significant (P<0.05) reduction in the activity and expression both of the inducible NOS (iNOS) and of cyclooxygenase-2 in lung tissue of pleurisy model. Methylguanidine is also able to reduce the appearance of nitrotyrosine and of the nuclear enzyme poly(adenosine diphosphate [ADP]-ribose) synthase immunoreactivity in the inflamed lung tissues. Treatment with aminoguanidine, the reference drug, significantly reduced all the evaluated pro-inflammatory parameters in carrageenan-treated rats. Taken together, the present results demonstrate that methylguanidine exerts potent anti-inflammatory effects that could be, in part, related to an inhibition of the expression/activity of the iNOS and cyclooxygenase-2 and, another part, may be related to a reduction of TNF-alpha release.