Alterations of hemodynamics and contractility were studied in 6 patients in whom two consecutive runs of supraventricular tachycardias (SVT) were electrically induced. Following initiation there was an abrupt decrease of arterial blood pressure, left ventricular systolic pressure, dp/dtmax, cardiac index and stroke work index (SWI). These parameters -- with the exception of SWI -- increased in the course of the SVT, however, control values were not reached. Mean pulmonary artery pressure steadily increased and remained on an elevated level until the tachycardia was stopped. When the SVT was electrically terminated, there was a transient overshoot of mean arterial and left ventricular systolic pressure, while mean pulmonary artery pressure slowly returned to control values. When the tachycardia was stopped by i.v. infusion of N-)diethylamino-3-propyl)-N-phenyl-indanamine-2 (aprindin, Amidonal) -- started at the 7th minute -- there was no statistically significant difference, neither during the SVT nor after cessation as compared to the values of the first run. Only mean pulmonary artery pressure remained at a higher level after SVT was stopped. It may, therefore, be concluded that in patients with otherwise normal hearts aprindin applied in a dosage sufficient to stop the SVT, does not exhibit noteworthy inotropic side effects.