Thirty-eightp atients with Wolff-Parkinson-Whsityen drome (WPW; type A 24 and type B 14), three patients with Lown-Ganong-Levine-syndro (mLeG L) and thirty-nine patients showing normal P-R intervali n sinus rhythm with episodes of paroxysmatla chycardia (n ormalP -R group) were examined during the following study.Paroxysmal supraventricular tachycardia (PSVT) was induced in 37 cases by a single electricalr ight atrial stimulation or by extra-stimulusm ethod: 24 normal P-R, 11 WPW and 2 LGL cases. Among 12 cases, in which PSVT was not induced, paired atrial stimulation successfullyin ducedP SVT in 2 normal P-R and 2 WPW cases.After intravenous atropine sulfate administration in the remaining 39 cases, PSVT was induced in 7 normal P-R and 3 WPW cases by a single atrial stimulation, and 1 normal P-R and 5 WPW cases by paired atrial stimulation. In total, PSVT was induced in 34 of the 39 normal P-R (87.2%), 16 of the 24 WPW-A (66.7%), 5 of the 14 WPW-B (35.7%) and 2 of the 3 LGL (66.7%) cases.Of the 30 WPW and LGL cases where spontaneousP SVT was recorded, P SVT was induced in 22 (73.3%), whereas PSVT was induced in only one of the 11 cases (9.1%) without any recorded attack.InducedP SVT terminateds pontaneouslyw ithin 3 minutes in 14 of 55 cases (25.5%) and was eliminated by a single or rapid atrial stimulation in all of the remaining 41 cases.The His bundle electrographics tudies revealed that the A2 -H2 interval in induced PSVT was significantlyl onger than the A1 -H1 interval in the basic cycle in all cases, and longer than the A2′-H2′ interval when PSVT was not induced. In WPW-A, the heart rate during induced PSVT was significantlyh igher after administrationo f atropinet han before, probably due to the shorteningo f A-H interval. The A2 -H2 interval was significantlylo nger in normal P-R cases having more frequent episodes than it was in those having less frequent episodes.The R-R interval during PSVT bad a significantc orrelationt o both effectivea nd functional refractory periods of A-V node in normal P-R cases and also to the effective refractory period of accessory pathway in WPW.Easier ind-uctitionf PSVT by paired stimulationc an be explained by the findingst-h at the atrial refractotry periodw as shortened by the first atrial stimulus, making penetration of the second atrial slunulus into A-V nodal pathway easier and that the A2 -H2 interval was longer in comparisonw ith cases of a single, atrial stimulus.These results, indicate the usefulness of the artificial PSVT induction in clinical practice of diagnosis and treatment.