The effect of Ro 11-1781, a drug that affects calcium transport, in 10 patients with paroxysmal supraventricular tachycardia (PSVT), was studied by intracardiac recording and stimulation. The re-entry circuit involved an accessory pathway that conducted only in the ventriculo-atrial direction in 5 patients, and was confined to the A-V node in 5 cases. Prior to administration of Ro 11-1781 tachycardia could be initiated in all patients. An intravenous bolus of 2 mg/kg during PSVT terminated the tachycardia in all cases by blockade in the A-V node. Ro 11-1781 lengthened the A-V nodal conduction time as well as the functional and effective refractory period of the A-V node. The effective refractory period of the "fast" pathway was variably changed. After Ro 11-1781 the tachycardia zone was abolished in 3 cases, reduced in 3, increased in 3 and was converted to an echo zone in 1. The ability to sustain the PSVT was lost in one subject. The heart rate during PSVT was slowed following Ro 11-1781. Ro 11-1781 appears to be useful for the termination of PSVT, but its ability to prevent PSVT varies. Beneficial effects include abolition or narrowing of the tachycardia zone, loss of the ability to sustain PSVT and a reduction in heart rate during PSVT. The widening of the tachycardia zone may be harmful.