Oxygen free radicals may participate in a variety of pathological cardiac conditions which are associated with an increased incidence of arrhythmias. However, evidence that free radicals per se can alter the electrical function of the myocardium is not convincing. Physiological solutions containing 3 mM dihydroxyfumaric acid (DHF), a compound known to generate free radicals, were superfused over calcium-tolerant cells isolated from the adult canine ventricle. The time course for changes in transmembrane action potentials was monitored using conventional microelectrode techniques. Changes were observed which could be conveniently segregated into three stages. Initially during superfusion with DHF, the voltage of the action potential plateau became more positive and the action potential duration increased (stage 1). Continued superfusion was associated with the development of both early and delayed afterdepolarizations (stage 2), which occasionally produced triggered beats. Subsequently, some cells failed to repolarize beyond -40 mV following an action potential upstroke. In cells which maintained normal levels of resting membrane potential, early and delayed afterdepolarizations ceased concomitant with the development of an increasingly more negative plateau voltage. Action potential duration decreased and plateau potential "collapsed", eventually merging with the resting level of the membrane potential. Resting membrane potential then gradually depolarized to less than -40 mV and all cells became inexcitable within 6 to 20 min (stages 3). Exposure of cells to xanthine (2 mM): xanthine oxidase (0.01 U/ml), another system known to generate free radicals, produced similar results. Superfusion with DHF solutions containing either superoxide dismutase or catalase delayed the appearance and attenuated the development of the changes in the cardiocyte action potential. The results demonstrate that isolated cardiocytes exposed to free radical generating solutions can undergo changes in their electrophysiological activity that resemble those said to underlie disturbances of cardiac rate and rhythm in the clinical setting.