In this study, we have assessed the activation of the cannabinoid CB2 receptor (CB2-R) in a model of mouse myocardial ischemia/reperfusion (I/R). The results show that treatment of animals with WIN55212-2, a CB1/CB2-R agonist, given 30 min before induction of I/R, significantly reduced the extent of infarct size (IS) in the area at risk, as measured 2.5 h later, with almost a 51% inhibition observed at the dose tested of 3.5 mg/kg intraperitoneally (i.p.). The protective effect of WIN55212-2 was almost abolished by the selective CB2-R antagonist AM630 (1 mg/kg i.p.) and not affected by the selective CB1-R antagonist AM251 (3 mg/kg i.p.). The CB2-R antagonist administered alone produced a slight but significant (P<0.05) increase in IS compared with vehicle alone. The protection afforded by WIN55212-2 was paralleled by lower values of myeloperoxidase activity and interleukin-1beta and of the CXC chemokine ligand 8 into the injured tissue. In conclusion, we demonstrate for the first time that exogenous and endogenous CB2-R activation reduces the leukocyte-dependent myocardial damage associated with an I/R procedure.