Cardioprotective and antiarrhythmic effects of three beta-blockers with different pharmacological properties were investigated in 33 anesthetized dogs with a 2-h coronary occlusion. Dogs were divided into 4 groups and received physiological saline or one of the following drugs using a 10-min infusion at 25 min before the occlusion: saline or control (n = 12), propranolol (0.3 mg/kg, n = 7), bisoprolol (0.05 mg/kg, n = 7), and nipradilol (0.2 mg/kg, n = 7) groups. Blood pressure did not significantly differ among the 4 experimental groups throughout the entire observation period. On the contrary, the postocclusion change (fall) in heart rate from the preocclusion value was significantly (P less than 0.05-0.01) greater in the drug-treated groups than in the control group. Each of the beta-blockers effectively prevented the development of ventricular arrhythmias associated with the 2-h coronary occlusion. In terms of assessing a cardioprotective effect, the respiratory control index and rate of oxygen consumption in State III in mitochondria, and lysosomal enzyme activities (N-acetyl-beta-glucosaminidase or beta-glucuronidase) in myocardial tissues, all prepared from both ischemic and non-ischemic areas, were measured using the respective, established methods. The 2-h coronary occlusion induced a mitochondrial dysfunction and leakage of lysosomal enzymes in the control group, whereas each beta-blocker significantly (P less than 0.05-0.01) protected mitochondria against ischemia and prevented the lysosomal enzyme leakage. The results indicate that the antiarrhythmic effects of beta-blockers on ischemic myocardium are, at least in part, due to their cardioprotective action, and these effects appear to be unrelated to the ancillary pharmacological properties of these drugs.