ECG recording of spontaneous, neurally-mediated syncope is rare. We have observed ten patients who sustained 70 syncopal episodes in whom: (1) ECG monitoring recorded syncope caused by ventricular asystole (AV block, three patients; sinus arrest; seven patients); (2) syncope and the spontaneously observed arrhythmias were reproducible by carotid sinus massage, upright tilt test, or eyeball pressure; and (3) no discernable cause of precipitating factors were detected. Two patients had a history of cardiac disease and four patients had only mild nonclinical ECG or echocardiographic abnormalities. Syncopal episodes recorded during Holter monitoring were of sudden onset in four patients and preceded by prodromal symptoms in six patients. The maximum RR pause was 9.4 +/- 3.7 seconds (range 4.5-15). Electrophysiological evaluation was normal in seven patients. Slight sinus node dysfunction or atrioventricular conduction abnormalities were noted in three others. The clinical characteristics of spontaneous and induced episodes strongly suggest that increased vagal tone played a role in causing the spontaneous events. Vagal stimulation tests are useful for the diagnosis of syncope of unknown origin.