We used a genetic approach to determine whether increasing the level of A3adenosine receptors (A3ARs) expressed in the heart confers protection against ischemia without causing cardiac pathology. We generated mice carrying one (A3tg.1) or six (A3tg.6) copies of a transgene consisting of the cardiomyocyte-specific &agr;-myosin heavy chain gene promoter and the A3AR cDNA. A3tg.1 and A3tg.6 mice expressed 12.7±3.15 and 66.3±9.4 fmol/mg of the high-affinity G protein–coupled form of the A3AR in the myocardium, respectively. Extensive morphological, histological, and functional analyses demonstrated that there were no apparent abnormalities in A3tg.1 transgenic mice compared with nontransgenic mice. In contrast, A3tg.6 mice exhibited dilated hearts, expression of markers of hypertrophy, bradycardia, hypotension, and systolic dysfunction. When A3tg mice were subjected to 30 minutes of coronary occlusion and 24 hours of reperfusion, infarct size was reduced ≈30% in A3tg.1 mice and ≈40% in A3tg.6 mice compared with nontransgenic littermates. The reduction in infarct size in the transgenic mice was not related to differences in risk region size, systemic hemodynamics, or body temperature, indicating that the cardioprotection was a result of increased A3AR signaling in the ischemic myocardium. The results demonstrate that low-level expression of A3ARs in the heart provides effective protection against ischemic injury without detectable adverse effects, whereas higher levels of A3AR expression lead to the development of a dilated cardiomyopathy.