In 9 patients with a history of paroxysmal atrial fibrillation (AF), basket catheters with 64 electrodes were used to determine electrophysiologic aspects of onset and early minutes of AF in the right atrium. Mapping was performed using 32 bipolar recordings after induction of AF by programmed atrial stimulation. In 7 patients, registrations were obtained that allowed the onset of AF to be analyzed after the induction. The most common mode of induction was a conduction block and local fragmentation of electrical activation in the area anterior to the tricuspid valve; this was observed in 5 patients. Local cycle lengths (5 min) and correlation dimension of repolarization Dpw_De (2 min) was calculated in 7 patients. The recording sites of the basket catheter were fluoroscopically projected to the anatomic sites of the right atrium. The highest Dpw_Re dimensions as an expression of pronounced chaotic activity were found in the area anterior to the tricuspid valve.