The clinical history and 24 hour Holter monitoring of 749 patients without ECG appearances of ventricular preexcitation were compared with the results of electrophysiological investigations to determine whether supraventricular arrhythmias initiated during endocavitary electrophysiological investigations had any pathological significance. Endocavitary studies were undertaken to investigate symptoms of dizziness, syncope and/or conduction defects except in the group of paroxysmal junctional tachycardia (PJT) where the indication was investigation of a tachycardia (78 cases). In 544 patients (Group I) no arrhythmias were initiated. Thirty five patients (6.4%) had supraventricular tachycardia (SVT), atrial flutter (AFI), atrial tachycardia (PAT), atrial fibrillation (AF) or PJT. The anterograde Wenckebach point (AV) was over 200/min in 22 cases (4%). In 400 patients the Wenckebach point or the retrograde Mobitz II (VA') point was 170/min in 56 patients (14%). In 28 patients with spontaneous SVT in whom retrograde conduction was studied, 3 had a Wenckebach 200/min (17.7%) and 9 had a Wenckebach point (VA') greater than 170/min (32%). In 86 patients (Group II) paired atrial stimulation induced PJT. Seventy nine patients (91.8%) had PJT : AV was greater than 200/min in 19 cases (22%) and VA was greater than or equal to 170/min in 69 cases (80.2%). In 119 patients (Group III) a supraventricular tachycardia (other than PJT) was induced. Manipulation of the catheter in the atrium led to AF, AFI or PAT in 9 patients. Eight patients had SVT (80.8%), AV was greater than 200/min in one case (11.1%) and VA' greater than or equal to 170/min in 5 of the 7 cases in which it was measured (71.4%). Paired atrial stimulation induced atrial echos in 63 patients; 47 presented spontaneous SVT : AV was greater than 200/min in 7 cases (11.2%) and VA' greater than or equal to 170/min in 23 of the 60 patients investigated (38.3%). Paired atrial extrastimuli triggered AF or PAT in 18 cases : 16 cases (88.8%) had spontaneous SVT. AV was greater than 200/min in 3 cases (16.6%), VA' was greater than or equal to 170/min in 10 of the 17 cases investigated (58.8%) : 11 of these patients also had atrial echos. Fixed atrial stimulation (less than 200/min) triggered AF or AFI in 14 patients. Nine had spontaneous SVT (64.3%) : AV was greater than 200/min in 2 cases (14.2%) and VA' greater than or equal 170/min in 2 of the 10 cases studied (20%). Ventricular stimulation induced SVT in 15 patients, 14 of whom had SVT (92%).(ABSTRACT TRUNCATED AT 400 WORDS)