The aim of the present study was to investigate the role of poly (ADP-ribose) synthetase (PARS) in a model of acute local inflammation (zymosan-activated plasma (ZAP)-induced paw edema), in which the oxyradicals, nitric oxide and peroxynitrite, are known to play a crucial role. Injection of zymosan-activated plasma (ZAP) into the rat paw induced edema formation. The maximal increase in paw volume was observed at three hours after administration (maximal in paw volume: 1.29+/-0.09 ml). At this time point, there was a marked increase in neutrophil infiltration in the paw, as measured by an increase in myeloperoxidase (MPO) activity in the paw tissue (260+/-25 mU/100 mg wet tissue). However, ZAP-induced paw edema was significantly reduced in a dose-dependent manner by treatment with 3-aminobenzamide (3-AB) or nicotinamide (NIC), two inhibitors of PARS, at 1, 2, 3, 4 hours after ZAP injection. PARS inhibition also caused a significant reduction of MPO activity. The paw tissues were also examined immunohistochemically for the presence of nitrotyrosine (a footprint for peroxynitrite formation). At 3 h following ZAP injection, staining for nitrotyrosine were also found to be localised within discrete cells in the inflamed paw tissue. Treatment with PARS inhibitor prevented the appearance of nitrotyrosine in the tissues. Our results suggest that in paw edema induced by ZAP, inhibition of PARS exert potent anti-inflammatory effects.