We have assessed, whether six agents, that either inhibit free radical formation or scavenge free radicals once they are produced, can reduce the incidence of reperfusion-induced arrhythmias, whether a free radical-generating system (FeCl3 X adenosine diphosphate) can increase the incidence of reperfusion-induced arrhythmias, and whether "anti-free radical" interventions can reduce reperfusion rhythm disturbances caused by the addition of FeCl3 X adenosine diphosphate. With the isolated, perfused rat heart (n = 15 in each group), inclusion of L-methionine (1 and 10 mM), superoxide dismutase (2.5 X 10(4) and 1 X 10(5) U/liter), catalase (5 X 10(4), 5 X 10(5), and 1 X 10(6) U/liter), mannitol (50 mM), glutathione (10 microM), or desferrioxamine (150 microM) significantly reduced the incidence of reperfusion-induced ventricular fibrillation and, in many cases, the incidence of reperfusion-induced ventricular tachycardia. The mean duration of sinus rhythm during reperfusion was also increased significantly. Perfusion of hearts with boiled superoxide dismutase (1 X 10(5) U/liter) or boiled catalase (1 X 10(6) U/liter) did not decrease arrhythmias. Conversely, under conditions where, in the control group, the incidence of reperfusion arrhythmias was lowered by increasing perfusate potassium to 6.5 mM, the addition of the free radical-generating system FeCl3 X adenosine diphosphate (0.1 microM X 1 microM) to the perfusion fluid increased dramatically the incidence of reperfusion-induced ventricular fibrillation and tachycardia. Simultaneous perfusion with FeCl3 X adenosine diphosphate and superoxide dismutase (1 X 10(5) U/liter), catalase (1 X 10(6) U/liter), mannitol (50 mM), methionine (10 mM), or desferrioxamine (150 microM) again reduced the incidence of reperfusion-induced arrhythmias and increased the duration of normal sinus rhythm during the reperfusion phase. Thus, addition of six "anti-free radical" interventions reduced the incidence of reperfusion-induced arrhythmias, addition of a free radical-generating system increased the incidence of reperfusion-induced arrhythmias, and simultaneous perfusion of the hearts with FeCl3 X adenosine diphosphate and "anti-free radical" interventions again reduced reperfusion rhythm disturbances. These results are further evidence supporting the hypothesis that oxygen-derived free radicals play an important role in the genesis of reperfusion-induced arrhythmias.