The electrophysiologic properties of flecainide (FLC),a new potential antiarrhythmic agent, are poorly defined.In this studies,they were investigated by conventionai microelectrode technique in isotated quinea pig left atrial muscles.The concentration of FLC used were 0.1,0.3,1,3,10and 30μmol/L.FLC produced a concentration-dependent decrease in maximal rate of rise of phase of the action potential (Vmax),action potential amplitude(AMP)and overshoot(OS). Vmax was reduced by 80.1±9.4%(P>0.05)after 0.3μmol/L FLC,by 68.2±7.8% (P<0.05) after 3.0μmol/L FLC and by 57.8±4.8% (P<0.01) after 10.0 μmol/L FLC at stim ulation rare of 1 Hz. The corresponding values of Vmax for 3 Hz 95.3±2.2% (0.3μmul/L FLC,P>0.05),69.2±9.7%(3μmol/L FLC,P<0.05) and 47. 3±11.5%(10μmol/L FLC,P<0.01).The Vmax was decreased markedly by 1.0μmul/L PLC in the stimulation rates of 0.2,0 5,1,2 and 3.3 Hz,their values were 92.3±2.2%(P<0.05),89.2±2.6% (P<0.05),89.4±3.7%(P<0.05),89.1±1.9% (P<0.01)and 83.8±4.1% (P<0.01).The depression of Vmax by FLC was dose-and frequency-dependence, this may be a important cause of FLC in the supperision of atrialarrhythmias. It was found that FLC at the stimulation frequencies of 2 Hz and 3.3 Hz tended to lengthen the APD with time and dose-dependence in quinea pig left atrium,which is very bene ficial for the patients who suffered from the arrythmias evoked by ischemia and reperfusion. It was also proved that FLC at the different stimulation frequencies could recovered the APD which have been shortened towards normal with a frequency-dependence,this may be another reason of the drug in the inhibition of atrial arrhythmi as.The slow response current might be influenced by 0.1 to 30μmol/L FLC in guinea pig left atrium only at the stimulation frequencies of 2 Hz and 3 Hz,5 to 15 minutes followtng the agent.