Standard microelectrode technique was used to study the effect of external free radical generating system (EFR) on action potential of papillary muscle cells of 48 Wistar male rats. EFR were produced from activated oxygen systems: including hydrogen peroxide (H_2O_2), hydroxyl (OH~-) and superoxider (O_2~-). The experiment was divided into six groups: a). control, b). OH~-, c). O_2~-, d). H_2O_2, e). OH~-+catalase(CAT, 150U/ml, f). H_2O_2+CAT and the data were analysed on-line by a computer (Apple Ⅱ Doctor-851). The results showed: (1). a highly significant reduction in action potential amplitude in group OH~- (from 110.5?11.8mV to 98.0?10.4mV), H_2O_2 (from 112.3?13.2mV to 99.0?13.6mV) and O_2~+ (from 117.2?14.5mV to 103.8?13.2mV). (2). a significant decrease in maxium rate of depolariztion (Vmax) respectively (P