Abstract: Objective To compare cardiaotoxic effect with sparfloxacin, the effects of levofloxacin on cardiac action potentials were examined. Methods Action potentials were recorded from isolated papillary muscles of guinea pig right ventricles by using conventional microelectrode techniques. Results With a stimulation frequency of 1Hz, both APD50 and APD90 were prolonged at any used concentrations, While, at the concentrations over 10μmol/L, levofloxacin prolonged the APD50 and APD90. With the stimulation frequency decreased, the prolongation of APD90 caused by LVFX or SPX were increased，indicating an inverse frequency dependency. Conclusion: LVFX has less potency to prolong APD as compare to SPX .