SUN 1165 is a newly synthesized antiarrhythmic drug with class Ic properties in vitro.We assessed acute electrophysiologic and hemodynamic effects of SUN 1165 in 18 patientswith supraventricular tachycardia (SVT) [6 patients with AV nodal reentrant tachycardia (AVNRT), and 12 patients with AV reentrant tachycardia (AVRT)]. All patients werefree from organic heart disease. Electrophysiologic and hemodynamic studies were performedbefore and 60 min after a single oral administration of SUN 1165 (150 or 200 mg).SUN 1165 significantly shortened sinus cycle length, lengthened sinoatrial conductiontime (98±21 to 112±21 msec, P< 0.01), AH interval (83±11 to 100±15, P<0.01), HVinterval (42±8 to 63±17, P<0.001), and RV effective refractory period (220±28 to 231±27, P<0.05).Antdrograde conduction via accessory pathway (AP) was abolished in 6out of 7 patients. Retrograde conduction via AP and AV node were abolished in 9 out of12 and 2 out of 6 patients respectively. Induction of SVT was completely suppressed in9 out of 13 patients. By hemodynamic studies (10 patients), decrease of stroke volumeindex and increase of mean pulmonary arterial pressure were demonstrated. No adverseeffects were observed. In conclusion, oral SUN 1165 has potent electrophysiologic effectson impulse propagation through AV node and AP with mild negative inotropic action. Andthe suppression of SVT induction was mediated through predominant effect on retrogradeconduction.