Protease activated receptor-2 (PAR2) may have proinflammatory effects in some tissues and protective effects in other tissues. The role of PAR2 in in vivo myocardial ischemia - reperfusion has not yet been determined. This study tested the hypotheses that PAR2 activation with the PAR2 agonist peptide SLIGRL (PAR2 AP) reduces myocardial infarct size when given at reperfusion in vivo, and this cardioprotection involves the ERK 1/2 pathway. Anesthetized rats were randomly assigned to the following groups with 30 minutes of regional ischemia and 3 hours reperfusion: (1) Control; (2) Vehicle (DMSO); (3) PAR2 AP: 1mg/ kg;(4) scrambled peptide: 1mg/kg;(5) PD 98059 (PD), 0.3 mg/kg; (6) Ly 294002 (Ly), 0.3 mg/kg; (7) PD + PAR2 AP: PD 0.3 mg/kg 5 minutes before PAR2 AP; (8) Ly + PAR2 AP: Ly 0.3 mg/kg 5 minutes before PAR2 AP; (9) Chelerythrine (Chel): 5 mg/kg; (10) Chelerythrine + PAR2 AP. Activation of ERK 1/2, ERK 5, Akt and the downstream targets of ERK 1/2 (P90RSK and BAD) was determined by Western blot. PAR2 AP significantly reduced infarct size compared to Control (36 ± 2% vs 53 ± 1%, P<0.05). PAR2 AP significantly increased phosphorylation of ERK 1/2, p90RSK and BAD but not Akt or ERK 5. Accordingly, the infarct size sparing effect of PAR2 AP was abolished by PD (PAR2 AP: 36 ± 2% vs PD + PAR2 AP: 50 ± 1%, p < 0.05) and by Chel (chelerythrine + PAR2 AP:58% ± 2%) but not by Ly (PAR2 AP: 36 ± 2% vs Ly + PAR2 AP: 38 ± 3%, p > 0.05). Therefore, PAR2 activation is cardioprotective in the in vivo rat heart ischemia-reperfusion model, and this protection involves the ERK 1/2 pathway and PKC.