The electrophysiological properties of the atrial cells (duration of action potential and refractory period, conduction speed) are differently influenced by vagal and catecholamine stimulation: the first favorising phenomena of macroreentry, the second abnormal automatism and induced activity. In the normal heart, vagal tone is dominant at atrial level explaining that the clinical entity of paroxysmal vagal atrial fibrillation is observed in the absence of detectable cardiac disease in relatively young men with ECG appearances of alternating atrial flutter and fibrillation. Catecholamine-induced atrial fibrillation is usually observed with underlying cardiac disease. Besides the clinical terrain and appearances, the observation of the circumstances surrounding the triggering of atrial fibrillation and the features of the sinus rhythm and its variability before the induction of the episodes are the main factors which demonstrate the modalities of the influence of the autonomic nervous system. In cases resistant to treatment, the knowledge of these factors provides a guide to treatment, indicating which antiarrhythmic agents are poorly adapted or even harmful.