Ajmaline test (50 mg intravenously over 1 minute) and ergometric exercise test were performed in 24 patients with the Wolff-Parkinson-White syndrome. The effects of both tests on the conduction in the accessory pathway (AP) were correlated with the anterograde effective refractory period (ERP) of the AP. Ajmaline caused transient anterograde block in the AP in 15 patients, 14 with an ERP of more than 250 ms and 1 with an ERP of 250 ms. Ajmaline failed to abolish preexcitation in 9 patients, all with ERP of the AP of 250 ms or less. Exercise testing caused anterograde block in the AP in 5 patients, in 2 during exercise (ERP of 340 and 360 ms) and in the remaining 3 patients during the recovery phase (ERP of 600, 520 and 460 ms). Of the 10 patients with an ERP of 250 ms or less, ajmaline did not abolish preexcitation in 9 and exercise did not abolish it in any of them. Of the 7 patients with an ERP of 330 ms or more, ajmaline abolished preexcitation in 7 and exercise abolished it in 5 patients. Of the 7 patients with ERP of the AP more than 250 ms but less than 330 ms, ajmaline but not exercise abolished preexcitation in all. Thus, ajmaline and exercise testing can detect patients with short and long ERPs of the AP, respectively, and the effects of ajmaline and exercise tests combined may be useful in the noninvasive assessment of various ranges of ERP of the AP.