This study was conducted to clarify the relationship between myocardial endogenous adenosine concentration in the great cardiac vein during ischemia and reperfusion-induced ventricular fibrillation (VF) and the occurrence rate of ischemia and reperfusion-induced VF. Sixty anesthetized mongrel dogs were subjected to 10-min left anterior descending artery occlusion and reperfusion. Blood from the great cardiac vein was sampled for the determination of adenosine and catecholamine concentrations. Hemodynamic parameters (blood pressure, lift ventricular dp/dt, left ventricular end-diastolic pressure, and cardiac output) were monitored throughout the experimental period. During coronary occlusion and reperfusion, VF occurred in 25 dogs (VF group). Ventricular fibrillation did not occur in 35 other dogs (non-VF group). Adenosine concentration in the great cardiac vein before the coronary occlusion did not differ between the two groups. However, the adenosine concentration in the great cardiac vein of the non-VF group significantly increased compared with that of the VF group (2,343 vs 65 pmol/ml, P = 0.026) during coronary occlusion and reperfusion. In contrast, hemodynamic parameters did not differ between the two groups. The study results demonstrated that adenosine concentration in the great cardiac vein was higher in the non-VF group than in the VF group, suggesting that myocardial endogenous adenosine may have a role in the suppression of ischemia and reperfusion-induced VF in dogs.