Rapid pacing from the high right atrium was performed in 7 patients with atrial flutter in whom potentials with multicomponent high-frequency deflections were recorded at the high right atrium to examine the origin of these potentials during transient entrainment in atrial flutter. In all of the patients with relatively slow rapid pacing, the potentials were captured orthodromically through the atrial flutter reentry circuit with a long conduction time. With more rapid pacing, the potentials were split into 2 associated components: P1 and P2. P1 was captured antidromically with a short conduction time whereas P2 was captured persistently in an orthodromic direction through the reentry circuit with a progressively long conduction time. In 3 of the 7 patients, atrial flutter was converted into another atrial flutter by rapid pacing. During this other atrial flutter, the potentials at the high right atrium were split from the beginning to form double potentials: D1 and D2. During rapid pacing, D1 and D2 were fused, and D1 was captured antidromically whereas D2 was captured orthodromically through the reentry circuit. In sinus rhythm, the potentials at the high right atrium formed fractionated potentials. These findings suggest that 1) fractionated potentials may represent 2 atrial regions with different conductivity properties, 2) fractionated potentials may be able to change into double potentials, and 3) double potentials may be attributable to their conductivity properties rather than refractoriness.