Electrophysiological study of 26 patients with paroxysmal supraventricular tachycardia showed that the tachycardias were due to a circus movement using a lateral atrioventricular bypass tract as a retrograde limb of the circuit. The Wolff-Parkinson-White syndrome was overt in 19 cases and concealed in the remaining seven. Retrograde P wave polarity and morphology during tachycardia were studied and related to the location of the bypass tract on the left or right side of the heart. In all cases the P wave during tachycardia was located after the QRS complex. Of 19 cases with left bypass tract, the retrograde P wave in lead I was negative in 14, diphasic (−.+) in four, and indeterminate in one case. In lead V, the P wave was positive in 16 cases and indeterminate in three. Of seven cases with right bypass tract, the retrograde P wave in lead I was positive and bimodal in six cases and indeterminate in the remaining case, while it was negative and bimodal in lead V1 in all but one case. Hence, it may be concluded that analysis of retrograde P wave morphology during tachycardia, together with other electrocardiographs factors, can be useful in determining the type of circuit used in reciprocating tachycardia.