There is increasing evidence for interactions among adenosine receptor subtypes in the brain and heart. The purpose of this study was to determine whether the adenosine A_2a receptor modulates the infarct size-reducing effect of preischemic administration of adenosine receptor agonists in intact rat myocardium. Adult male rats were submitted to in vivo regional myocardial ischemia (25 min) and 2 h reperfusion. Vehicle treated rats were compared to rats preconditioned with the A1 agonist CCPA (10 µg/kg), the nonselective agonist NECA (10 µg/kg), or the A2a agonist CGS21680 (20 µg/kg). Additional CCPA and NECA treated rats were pretreated with the A1 antagonist DPCPX (100 µg/kg), the A_2a/A_2b antagonist ZM241385 (1.5 mg/kg) or the A3 antagonist MRS1523 (2 mg/kg). CCPA and NECA reduced myocardial infarct size by 50 and 35%, respectively, vs vehicle, but CGS2160 had no effect. DPCPX blunted the bradycardia associated with CCPA and NECA, while ZM241385 attenuated their hypotensive effects. Both the DPCPX and ZM241385 blocked the protective effects of CCPA and NECA. The A3 antagonist did not alter the hemodynamic effects of CCPA or NECA, nor did it alter adenosine agonist cardioprotection. None of the antagonists alone altered myocardial infarct size. These findings suggest that although preischemic administration of an A_2a receptor agonist does not induce cardioprotection, antagonism of the A_2a/A_2b receptor blocks the cardioprotection associated with adenosine agonist pretreatment.