Background: Previous studies suggest that a short temporal excitable gap exists between the fibrillation waves during atrial fibrillation (AF). The aim of this study was to investigate the role of that gap in the development of sustained AF in goats. Methods and Results: Eight female goats were instrumented with left atrium (LA) electrodes, and sustained AF (>24 h) was induced by intermittent rapid atrial pacing for 9.3±4.6 days. In the process of sustained AF development, the atrial effective refractory period (AERP), refractory period during AF (RP_AF), mean AF cycle length (AFCL), temporal excitable gap during AF (EG_AF = AFCL - RP_AF) and degree of fractionation of fibrillation electrograms at LA were studied. When the induced AF lasted for 3-10 min, AFCL, RP_AF and EG_AF were 98.3±11.0 ms, 90.5±13.2 ms and 7.8±2.4 ms, respectively. During sustained AF, the values were 84.9±5.2 ms, 63.0±4.8 ms and 21.9±3.5 ms, respectively (P<0.05). Percentage of single potentials was 94.2±3.9% and 75.6±5.5%, respectively (P<0.05). Conclusions: In this model progressive shortening of atrial refractoriness and widening of the temporal excitable gap induced by electrical remodeling created an electrophysiologic substrate for the perpetuation of AF. (Circ J 2010; 74: 655-663)