Electrocardiographic and dectrophysiologic studies were performed in 21 patients with atrioventricular (A-V) nodal reentrant tachycardia (AVNRT).Patients were classified into three types according to the patterns during the tachycardia, i.e., (1) type I; an atrial echo (Ae) occurred simultaneously with a ventricular electrogram (VE) (14 patients), (2) type II; an Ae occurred in front of the next VE (four patients) and (3) type III; an Ae followed immediately after the preceding VE (three patients). All patients with type I AVNRTshowed discontinuous A-V and continuous ventriculoatrial (V-A) conduction curves. "Enhanced V-A conduction" (1:1 V-A conduction at the ventricular paced rate of 200 beats/min) was demonstrated in five of the 14 and after verapamil in two of the five patients. In type II AVNRTthe tachycardia was initiated with a minimal amount of A-Vnodal conduction delay and second degree A-V or V-Ablock was observed in all four patients. V-Aconduction was poor in these patients. In type III AVNRT A-Vconduction curves were discontinuous and retrograde study revealed the presence of retrograde dual A-V nodal pathways. In conclusion, A-Vand V-Aconduction in patients with AVNRTwere variable and this variability seemed to be responsible for different patterns of AVNRT.