Rapid atrial pacing is a useful technique and often the therapy of choice to terminate atrial flutter in patients. However, interruption of atrial flutter by rapid atrial pacing may not always produce sinus rhythm, but rather may result in atrial fibrillation. Twelve patients with spontaneous atrial flutter that had been present for greater than 24 h were studied to assess the efficacy of atrial pacing, alone and in combination with procainamide, to convert atrial flutter to normal sinus rhythm. Rapid atrial pacing for greater than or equal to 15 s from selected atrial sites at selected pacing rates were performed during atrial flutter. The initial pacing rate was always at a cycle length 10 ms shorter than the atrial flutter cycle length. If atrial flutter persisted after cessation of pacing, it was repeated at progressively shorter cycle lengths until either a rate of 400 beats/min was achieved or atrial fibrillation was induced. In two patients, atrial flutter was converted to sinus rhythm with pacing alone. Three patients developed sustained atrial fibrillation as a result of the rapid atrial pacing, this rhythm ultimately reverting back to atrial flutter in two. Ten patients received procainamide and 9 of the 10 had lengthening of the atrial flutter cycle length by a mean of 68 ms (1 patient continued to have atrial fibrillation). Then, using the same atrial pacing protocol, high right atrial pacing alone at a mean cycle length of 227 ms interrupted atrial flutter in all these patients, returning their rhythm to sinus rhythm. It is concluded that intravenous procainamide effectively augments the efficacy of rapid atrial pacing to convert atrial flutter to sinus rhythm.