Electrophysiologic studies were performed in a 41 year old man for analysis of paroxysmal tachycardias appearing in various electrocardiographic patterns of supraventricular and ventricular bigeminy, junctional and ventricular tachycardia and atrial fibrillation, among others. All these arrhythmias were due to dual atrioventricular (A-V) nodal pathways with simultaneous dual fast and slow conduction of single atrial beats at a normal basic sinus rate. Moderate changes in sinus rate and in fast or slow pathway conduction times, or both, changed the position of the slowly conducted beats between the neighboring two fast conducted beats and resulted in various electrocardiographic manifestations of the conduction disturbance. Different blocks, such as second degree type 1, 2:1, 3:1 and possibly also type II, in one of the two pathways and occasionally aberrant conduction induced even more unusual tracings. After intravenous injection of 25 mg of ajmaline, unexpected lengthening and shortening of the A-H interval occurred, suggesting variable shifts between fast and slow pathway conduction. The incidence of dual A-V nodal pathways is discussed; it was documented in 17 (4.2 percent) of 405 patients studied. A theoretical model of A-V nodal conduction is proposed to explain its normal properties and abnormal patterns.