Background and objectives: Non steroidal anti inflammatory drugs (NSAIDs) are widely used to reduce inflammation, pain and fever. The present study was organized to induce an experimental inflammation in an animal model, using a putative biological oxidant, Peroxynitrite (ONOO-) and to study the effects of Flunixin meglumine and Ketoprofen on the pressure-induced pain. Materials and Methods: For this purpose, 24 male guinea pigs were divided into 4 groups each consisting of 6 animals. Three groups (groups 2, 3 and4) were injected Peroxynitrite and one group (the first: control) received physiological salt solution subcutaneously in the paw. After induction of a local inflammatory response, Flunixin meglumine (1mg/kg) and Ketoprofen (2 mg/kg) were injected to the second and third groups, 5 times with 12h intervals. The first and the fourth groups were injected saline solution in the same manner. Pressure analgesiometry was performed before and 1 hour after injections. Result: The animals in all 3 groups treated with Peroxynitrite demonstrated an increased sensitivity to painful pressure (P<0.05). Both NSAIDs decreased the pain sensation dramatically after the 1st and the 2nd injections but, not after the 3rd, 4th and the 5th injections. Conclusion: The study suggested that NSAIDs may be helpful in reducing pressure-induced pain in animal model in early hours of the treatment, whereas the effect subsides over time and ends up after a few days. This effect may be of importance in humans who receive these kinds of drugs for a long period as they may not be effective in reducing pain after a while.