The electrophysiologic properties of left-sided accessory pathways (APs) were examined by cardiac stimulation in 55 patients with Wolff-Parkinson-White syndrome. Atrioventricular and ventriculoatrial conduction times were assessed at the coronary sinus level nearest to the AP and then plotted graphically as a function of coupling interval (for atrial and ventricular refractory period determinations). Of 29 patients with anterograde conduction over the AP, 10 (34%) exhibited decremental conduction. However, only two (7%) had a maximal decrement equal to or more than 30 msec. In the other eight (27%) patients the maximal decrement ranged from 10 to 20 msec. The longest coupling interval at which anterograde decremental conduction was demonstrated ranged from 260 to 440 msec (346 +/- 52 msec). The shortest coupling interval ranged from 240 to 320 msec (265 +/- 24 msec). The anterograde decremental conduction zone was 91 +/- 55 msec. Of 51 patients with retrograde conduction over the AP, 23 (45%) exhibited decremental conduction. However, only eight (15%) had a maximal decrement equal to or greater than 30 msec. In the other 15 (29%) patients the maximal decrement ranged from 10 to 25 msec. The longest coupling interval was 338 +/- 70 msec. The shortest coupling interval was 275 +/- 42 msec. The retrograde decremental conduction zone was 72 +/- 47 msec. There was a significant inverse correlation between the AP effective refractory period and the maximal decrement (r = -0.42; p < 0.05). The comparison of maximal ventriculoatrial conduction time with the maximal decrement revealed a positive correlation (r = 0.63; p < 0.01). These data reveal that minimal decremental conduction over left-sided APs is not an uncommon finding and stress that care should be taken in evaluation of conduction over these connections.