Sulamserod is a 5-HT4 receptor antagonist, with antiarrhythmic activities. In Vivo: Sulamserod (30 mg/kg) prolongs mean ERP (11568 versus 14667 ms, P,0.01) and wavelength (8.360.9 versus 9.960.8 cm, P,0.01), reduces dispersion of ERP (1565 versus 861 ms, P,0.01), and minimally slows conduction velocity (7264 versus 6765 cm/s, P,0.01) in pigs. Sulamserod produces no ventricular electrophysiological effects. Sulamserod terminates atrial flutter in 6 of 8 animals and atrial fibrillation in 8 of 9 animals and prevents reinduction of sustained tachycardia in all animals^[1].