Objective To measure the effects of class Ⅰ antiarrhythmic agents, procainamide, Lidocaine and propafenone on Na+/Ca 2+exchange tail current. Methods Using whole-cell patch clamp technique in isolated guinea pig ventricular myocytes with an enzymatic procedure, Na+/Ca 2+exchange tail current were measured respectively before and after application of drugs. Results All of the three drugs concentration-dependently inhibited Na+/Ca 2+exchange tail current, but there were different degrees of inhibition among them, the effect of procainamide was the most. Procainamide 10 μm, 50 μm, 100 μm decreased the Na+/Ca 2+, tail from (142±13) pA to (132±11) pA, (120±12) pA, (96±13) pA, respectively; Lidocaine 10 μm, 50 μm, 100 μm inhibited the Na+/Ca2+, tail from (155±10) pA to (150±9) pA, (141±9) pA, (132±9) pA, respectively; and propafenone 10 μm, 50 μm, 100 μm from (151±8) pA to (134±8) pA, (119±10) pA, (93±11) pA, respectively. Conclusion ClassⅠantiarrhythmic agents had inhibitory effects on Na+/Ca 2+exchange tail current in isolated guinea pig ventricular myocytes, and the extents of effect were different among drugs.