Animal experiments have confirmed that a short but variable excitable gap is present during induced atrial fibrillation. The existence of this gap allows atrial fibrillation to be locally captured by rapid pacing. An area of up to 4 cm in diameter can be activated by uniform wave fronts propagating away from the site of stimulation, although the size of this area may be limited by intra-atrial conduction block or by collision with fibrillation waves. Rapid pacing can accelerate atrial fibrillation by inducing local re-entry circuits with a cycle length shorter than the pacing interval. During pacing-induced atrial fibrillation in patients undergoing surgery for Wolff-Parkinson-White syndrome, three types of atrial activation have been identified, which vary in their degree of complexity and in the length of their fibrillation intervals. It appears that the spectrum of varying cycle lengths during atrial fibrillation might be explained by a mixture of different sorts of atrial re-entry patterns, comprising purely functional intra-atrial re-entry, random re-entry, anatomical re-entry, and nodal re-entry.