Electrophysiologic investigations have been performed in 13 patients with symptomatic WPW syndrome. The effective refractory periods of the right atrium (AERP), the antegrade effective refractory periods of the accessory pathway (AP-ERP) during sinus rhythm and at different cycle length, the shortest RR-intervals with preexcitation during atrial fibrillation (RR-AF) and the appearance of block in the AP during incremental atrial pacing have been measured. The data collected during sinus rythm (SR) and at different pacing rates were analyzed and compared. During SR (cycle length 851 +/- 157 ms) AERP measured 263 +/- 56 ms and AP-ERP 309 +/- 55 ms. At a cycle length of 450-550 ms, AERP measured 221 +/- 36 ms and AP-ERP 250 +/- 35 ms, and at a cycle length of 400-450 ms AERP was 213 +/- 35 ms and AP-ERP 230 +/- 42 ms. The mean RR-AF was 253 +/- 35 ms. A significant correlation was found between AERP and AP-RP at a cycle length of 400-450 ms (r = 0.92), but not between AP-ERP and RR-AF at any cycle length. Block in the AP occurred at a mean pacing cycle length of 280 +/- 42 ms. The electrophysiologic measurements of patients with syncope did not differ from patients without syncope in our group. The data show that in patients with a long AP-ERP (greater than 290 ms) during SR an increase of the heart rate leads to a significant shortening of the AP-ERP. No relevant correlation was found between AP-ERP and mean RR-AF. Patients with AP-ERP shorter than RR-AF appear to be a high-risk subgroup for ventricular fibrillation during AF.