Study on the genesis of the double potential recorded in the high right atrium in atrial flutter and its role in the reentry circuit of atrial flutter. To investigate the genesis of the double potential (DP), which is two separate waves, and its role in the reentry circuit of atrial flutter (AF), we performed overdrive pacing (ODP) from the high right atrium (HRA) in six cases of spontaneous AF in which the DP was recorded in the HRA. In four of the six cases, when the DP was arbitrarily designated D1 and D2, D1 and D2 showed progressive fusion during ODP. In addition, the D1 return cycle, immediately after the termination of ODP, corresponded to the AF cycle, and the D2 return cycle corresponded to the pacing cycle. This may indicate that the DP is caused by the collision of two directional waves. Furthermore, it is suggested that the HRA plays an important role in preventing a possible shortcutting of reentry waves and in stabilizing the reentry circuit of AF. 