In this study, using electron spin resonance (ESR), we investigated the relation at the time of reperfusion between free radicals originating from the mitochondria of the canine myocardium and arrhythmias induced by reperfusion as well as the effect of radical scavengers on both. The left anterior descending artery was ligated just below the first diagonal branch and then reperfused for 10 minutes in 48 adult mongrel dogs. The dogs were divided into six groups consisting of: 1) control group administered no radical scavengers (n = 8), 2) SOD group (n = 6) receiving superoxide dismutase (15,000 U/kg), 3) SOD + CAT group (n = 6) receiving SOD (15,000 U/kg) and catalase (45,000 U/kg), 4) L-SOD group (n = 6) receiving liposomal-encapsulated SOD (30,000 U/kg), 5) CV-3611 (2-O-octadecylascorbic acid) group (n = 8) receiving CV-3611 (10 mg/kg), and 6) CoQ10 group (n = 6) receiving coenzyme Q10 (10 mg/kg). SOD, SOD + CAT, L-SOD, CV-3611, and CoQ10 were administered into the left atrium prior to reperfusion. The second lead of the electrocardiogram was continuously monitored during the experiment. The following results were obtained. 1) The relative intensity (RI) of the electron spin resonance signal of the mitochondria of the reperfused portion of the myocardium was smaller (p less than 0.025) in the SOD, SOD + CAT, L-SOD, CoQ10 groups (1.08 +/- 0.36, 0.92 +/- 0.19, 0.91 +/- 0.11, and 0.81 +/- 0.09, respectively) than in the control group (1.70 +/- 0.20).(ABSTRACT TRUNCATED AT 250 WORDS)