This study was undertaken to establish the nature (intrinsic or extrinsic) of sinus node dysfunction in patients presenting with syncope and abnormal response (sinus pause greater than 3'') to vagal manoeuvres (carotid sinus massage and/or eye-ball compression). To this purpose 29 patients (20 males, 9 females, aged 18-79 yrs, mean = 60) underwent an electrophysiologic study. In all we measured before and after autonomic blockade with Propranolol (0.2 mg/kg) and Atropine sulfate (0.04 mg/kg) the following parameters: sinus rate, corrected sinus node recovery time and sino-atrial conduction time. According to the presence or absence of electrocardiographic signs of sinus node dysfunction the patients were divided into three groups: Group A: 11 patients without electrocardiographic signs of sinus node dysfunction; Group B: 13 patients with borderline electrocardiographic signs of sinus node dysfunction (sinus bradycardia between 40 and 60 beats/min); Group C: 5 patients with definite electrocardiographic signs of sinus node dysfunction (sinus bradycardia less than or equal to 39 beats/min and/or sinoatrial block and/or sinus arrest). Fifteen patients (52%) had signs of organic heart disease. The main results obtained were: At least one electrophysiological test (sinus rate, corrected sinus node recovery time or sino-atrial conduction time) was abnormal in 10 patients (34,5%) before autonomic blockade and in 8 patients (27%) after autonomic blockade. Two of these 8 patients belonged to Group A (18%), 1 to Group B (8%) and 5 to Group C (100%). Six of these 8 patients were suffering from an organic heart disease and 2 were not.(ABSTRACT TRUNCATED AT 250 WORDS)