The purpose of this study was to determine the relationship between ventricular excitability and the extrastimulus (S2) coupling interval in early diastole in humans. The influence of the electrode configuration during bipolar stimulation was examined. Ventricular strength-interval curves were constructed using an eight beat drive train at a cycle length of 600 msec, 2-msec decrements in the S2 coupling interval, and 0.2-mA increments in the current intensity of S2 at each coupling interval. A transient dip in the excitation threshold was observed in early diastole during bipolar pacing when the distal electrode was the anode in six of 17 patients (35%). In contrast, this type of dip was not observed when the distal electrode was the cathode. The excitation threshold at the trough of the dip ranged from 1.0 to 2.2 mA. Unipolar strength-interval curves indicated that a dip occurred with anodal but not cathodal pacing. We postulated that the dip might interfere with the accurate determination of the ventricular effective refractory period by resulting in transient loss of ventricular capture during decremental scanning of diastole with S2. A gap in ventricular capture was produced in all six patients who demonstrated a dip in the bipolar strength-interval curve by selecting an S2 current intensity that fell within the dip. In conclusion, the bipolar strength-interval curve may display an early diastolic dip in the excitation threshold, depending on the electrode configuration.(ABSTRACT TRUNCATED AT 250 WORDS)