The electrophysiological effects of anti-arrhythmic drugs in man may be classified in three groups: -- Group I: comprising drugs whose characteristic action is on the AV node (beta blockers, verapamil, digitalis) The nodal conduction time (A-H interval) and refractory periods are increased. -- Group II: comprising drugs acting on the His-Purkinje system, the AV nodal conduction staying unchanged. This group has two sub-groups. Sub-group A: these drugs delay the His-Prukinje conduction (increased H-V interval). Examples are quinidine, procainamide, disopyramide, ajmaline, chloro-acetyl-ajmaline. In addition these drugs usually increase the atrial refractory periods and those of accessory pathways. Sub-group B: the His-Purkinje conduction is unchanged but the refractory periods are modified: lengthened (bretylium tosylate) or shortened (diphenylhydantoin, lignocaine, mexiletine). -- Group III: which includes amiodarone and aprindine whose effects are mixed: on the one hand AV nodal depression, and on the other, alteration of the His-Purkinje conduction manifested by an increased H-V internal (aprindine) or refractory periods (amiodarone). These preparations also increase the refractory periods of accessory AV pathways and amiodarone increase the refractory periods of the atria. This type of classification could help towards a more rational clinical approach to the use of anti-arrhythmic drugs.