In 10 patients with supraventricular tachycardia who underwent an electrophysiological study the ectopic atrial origin of the tachycardia could be demonstrated by the intracavitary localization of the earliest depolarized point (six cases in the right atrium and four cases in the left). Vagal stimulation and/or the i.v. infection of ATP induced or increased the degree of AV block during tachycardia; in five cases ATP interrupted the tachycardia. On the basis of their clinical and electrophysiological behavior, the patients were divided into two groups. In the first one (6 patients) tachycardia was persistent, started following late atrial beats with the same morphology of the subsequent and showed a progressive initial rate increase (warm-up phenomenon); programmed atrial stimulation failed to start and interrupt it. In the second group (4 patients) tachycardia was paroxysmal or repetitive, started following early atrial beats and could be induced and interrupted by programmed atrial stimulation. With respect to the electrophysiological characteristics of each group the authors try to define the possible mechanism responsible for the tachycardia (automaticity and micro-reentry).