Thirteen patients with paroxysmal atrial tachyaarhthmias (six atrial supraventricular tachycardia, six atrial fibrillation, and one atrial flutter) have been investigated by recording of monophasic action potentials (MAP) during a period free from arrhythmias. None of the patients differed from healthy individuals with respect to any of the MAP variables. Occasional recordings showed values not found in healthy individuals, such as prolonged duration of the MAP or slow phase 4 depolarization. The atrial effective refractory period (AERP) was determined in two patients. One AERP was short compared with values found in healthy individuals. Thus some MAP recordings gave a possible explanation for the mechanism behind the arrhythmias, while other recordings showed no differences from results in healthy individuals.