To elucidate differences in conduction properties among the normal atrioventricular (AV) node and the antegrade fast and slow dual AV nodal pathways (DAVNPW), AV nodal conduction curves were analyzed quantitatively in 38 patients. Eighteen patients had antegrade DAVNPW with AV nodal reentrant tachycardia (AVNRT) (dual pathways group) and the remaining 20 had smooth AV nodal conduction curves, without evidence of AV nodal dysfunction (control group). The effective refractory period (ERP) of the antegrade fast pathway was longer than that of the normal AV node (at both basic cycle lengths of 700 and 500 msec, p less than 0.01). Although the atrial premature beats were delayed by a longer ERP in the fast pathway, there was no significant difference in the degree of prolongation of AV nodal conduction time related to shortening of the coupling interval (i.e., ratio of A2H2 increment to A1A2 decrement) between these two pathways. On the other hand, the ERP of the antegrade slow pathway was similar to that of the normal AV node. The degree of prolongation of AV nodal conduction time (relative to the shortening of the coupling interval) was greater in the antegrade slow pathway than in the normal AV node. In conclusion, these findings suggest that in DAVNPW with AVNRT: (1) the antegrade fast pathway is similar to the AV node and its conduction properties are unlikely to be better than those of the normal AV node and (2) the antegrade slow pathway has quantitatively poorer conduction properties than the normal AV node, since it has a greater degree of decremental conduction.