Fourteen patients with permanent electrocardiographical features of the Wolff-Parkison-White syndrome in sinus rhythm referred for electrophysiological investigation also underwent maximal exercise tolerance tests. The working hypothesis was that in patients with the Wolff-Parkinson-White syndrome with accessory pathways of longer effective refractory periods than the normal pathway (group I) the delta wave should disappear on exercise, whilst in patients with accessory pathways with shorter refractory periods than the normal pathway (group II) the delta wave should persist. Of the 9 patients in group I,the delta wave regressed in 8 and persisted in 1 patient; of the 5 patients in group II, the delta wave persisted in 4 of them. Three patients had attacks of tachycardia during or just after the exercise tolerance test. These results suggest that the exercise tolerance test may help in the identification of patients with accessory pathways with long refractory periods, less susceptible to rapid ventricular rhythms should atrial fibrillation occur, and therefore with better prognoses.