Animal models are used to research the mechanisms of pain and to mimic human pain. The purpose of this study was to determine the degree of interaction between dexketoprofen and dexibuprofen, by isobolographic analysis using the formalin orofacial assay in mice. This assay presents two-phase time course: an early short-lasting, phase I, starting immediately after the formalin injection producing a tonic acute pain, leaving a 15 min quiescent period, followed by a prolonged, phase II, after the formalin and representing inflammatory pain. Administration of dexketoprofen or dexibuprofen produced a dose-dependent antinociception, with different potency, either during phases I or II. The co-administration of dexketoprofen and dexibuprofen produced synergism in phase I and II. In conclusion, both dexketoprofen and dexibuprofen are able to induce antinociception in the orofacial formalin assay. Their co-administration produced a synergism, which could be related to the different degree of COX inhibition and other mechanisms of analgesics. (C) 2010 Elsevier Inc. All rights reserved.