The conduction time over the lower common pathway (LCP) in AVNRT can be assessed by subtracting the HA-interval during tachycardia (HA(t)) from that during ventricular pacing at exactly the same cycle length (HA(p)) (Delta HA = HA(p)-HA(t)). It has been suggested that H-A measurements may help in the differentiation, of Slow/Fast from Slow/Slow AVNRT. This study evaluated prospectively in 61 consecutive patients with AVNRT (43 +/- 15 y; 46 women, all with antegrade conduction during AVNRT over the slow pathway) how often a reliable measurement of the length of the LCP could be made, and in how far the results were concordant with mapping criteria for the differentiation of Slow/Fast from Slow:;Slow AVNRT. A new para-Hisian pacing technique (using only the His bundle catheter) was applied in all patients. Comparison of HA(t) and HA(p) was possible in 44 of the 61 patients (72%). In these 44 patients HA(p), was longer than HA(t) in 12 patients, indicating the presence of a LCP. All patients with Delta HA greater than or equal to 15 ms had earliest retrograde atrial activation in the posterior septum (Slow/Slow AVNRT; n = 6) or simultaneously in the anterior and posterior septum (n = I). On the other hand, 31 of the 32 patients without evidence of a substantial LCP (Delta HA less than or equal to 0) had typical Slow/Fast AVNRT. Moreover, although it appears logical for Slow/Fast AVNRT to have a shorter HA(t) than Slow/Slow AVNRT, an HA(p) of greater than or equal to 70 ms was a better discriminator between the two forms of AVNRT than any HA(t) value. Therefore, Delta HA greater than or equal to 15 ms !(sens. greater than or equal to 86%; spec. greater than or equal to 97%) or HA(p) greater than or equal to 70 ms(sens. = 100%; spec. 89%) were,, highly indicative for the Slow/Slow variant of AVNRT. Using a para-Hisian pacing technique, H-A measurements can be performed in 72% of AVNRT patients?. They can be used as an important tool in the differentiation of Slow/Fast and Slow/Slow AVNRT.