Recent studies of the mode of initiation of atrial fibrillation (AF) have emphasises the role of the autonomic nervous system, vagal and sympathetic, in the development of this arrhythmia. Continuous 24 hour electrocardiographic recording (Holter monitoring) is a good method of studying arrhythmias. The role of vagal or sympathetic stimulation was evaluated by retrospective analysis of 3 000 Holter recordings performed in our department. Three groups of patients were identified: Group I, patients with bradycardia at the time of onset of AF (sinus rhythm less than 60/min); Group II, patients with sinus rhythms greater than or equal to 60/min at the time of onset of AF. Some of the rapid atrial arrhythmias encountered in the sick sinus syndrome may be confused with AF on Holter monitorings; patients with signs of sinus node dysfunction were therefore set apart (Group III). The following features were studied in all patients: the morphology of the sinus P wane, the duration of the PR interval, the permeability of the atrioventricular node at the start of AF, the presence of arrhythmias before AF and the time of day of the episodes of AF. The highest incidence of prolongation and abnormalities of the P wave and increased AV conduction at the beginning of AF was observed in Group I. There was also a marked prevalence of supraventricular beginning before AF and most episodes of AF were observed during the night. In groups II and III no attacks of AF were observed at sinus rhythm greater than 94/min. The extrasystole initiating AF had the same average coupling interval in all three groups.(ABSTRACT TRUNCATED AT 250 WORDS)