We studied the electrophysiologic characteristics of atrioventricular (AV) nodal conduction in patients with reciprocating tachycardia (RT) without ventricular preexcitation, and the relation of these characteristics to RT cycle length (CL). Thirty-five symptomatic patients who had a normal PR interval (0.13-0.20 second) during sinus rhythm underwent detailed intracardiac electrophysiologic study during which ventricular preexcitation was excluded, and the RT mechanism was determined. RT was due to reentry using an accessory AV pathway capable of conduction only in the retrograde direction (concealed AP) in 13 patients (37%) and to reentry within the AV node in 22 (63%). Dynamic properties of AV conduction (assessed by degree of AH prolongation during progressive increase in atrial pacing rate) were normally distributed (p < 0.005); 12 patients (34%) fulfilled the criteria for enhanced AV conduction (EAVC). The patients with EAVC had a shorter RT CL than did patients without EAVC (294 ± 43.3 msec vs 360 ± 68.1 msec, p < 0.01). However, CL differences were primarily due to the influence of in the subgroup of patients with RT using a concealed AP (EAVC CL, 274 ± 35.1 msec; without EAVC, 326 ± 15.7 msec, p < 0.005). The RT CL in patients with reentry within AV node was not measurably influenced by concomitant EAVC (EAVC, 314 ± 43.8 msec; without EAVC, 376 ± 76.8 msec) (NS). This study suggest that despite the presence of a normal PR interval during sinus rhythm, dynamic AV conduction responses can vary widely in patients with RT. In patients with RT using a concealed AP, but not in those with reentry within the AV node, coexisting diminished physiologic AV conduction slowing may be associated with more rapid tachycardia rates.