To assess whether free radicals affect the characteristic sequence of events in an in vitro model of ischemia and reperfusion, isolated canine cardiac Purkinje fibers were exposed to anti-free radical agents, superoxide dismutase 50 U/ml; catalase 600 U/ml; mannitol 2 mM and 20 mM and combined superoxide dismutase, catalase, and mannitol 20 mM. Fibers were superfused for 60 min with altered Tyrode's solution which mimicked some conditions of ischemia (glucose-free, containing lactate, equilibrated with 90% N2-10% CO2), and then re-exposed to normal, oxygenated Tyrode's solution. Anti-free radical agents alone had no electrophysiological effects. Ischemic conditions resulted in moderate depolarization in most preparations (10 control; 43 treated) but in severe depolarization, to less than -60 mV, in some (3 control, 11 treated). Re-exposure to normal, oxygenated solution resulted in prompt initial hyperpolarization followed by marked depolarization, and finally by recovery to baseline maximum diastolic potential. Anti-free radical agents had not effect on maximum diastolic potential during exposure to ischemic conditions or on initial hyperpolarization following re-exposure to oxygenated solution. Preparations that became severely depolarized during exposure to ischemic conditions were not protected by anti-free radical interventions from developing marked depolarization following re-exposure to oxygenated solution. However, in fibers only moderately depolarized by the ischemic conditions, all anti-free radical interventions except mannitol, 2 mM significantly attenuated depolarization following re-exposure to oxygenated solution. Anti-free radical interventions significantly reduced the number of preparations that became markedly depolarized (to less than -65 mV) during re-exposure to oxygenated solution (except for mannitol, 2 mM). Agents had no effect on the occurrence of abnormal automaticity; the occurrence of inexcitability was reduced, but not significantly, except when all agents were used in combination. The results suggest that free radical production may contribute to electrophysiological abnormalities when Purkinje fibers are re-exposed to oxygenated Tyrode's solution after they have been exposed to ischemic conditions.