The increase of the ventricular fibrillation threshold (VFT) by 711389-S, a new antiarrhythmic agent, was compared with several other antiarrhythmic drugs using both in vitro (Langendorff) and in vivo preparations of guinea-pigs. The doses of antiarrhythmic drugs for increasing VFT by 10 volts in in vitro (microgram) and in vivo (mg/kg, i.v.) preparations were as follows: 711389-S: 4.2, 0.15; disopyramide: 32.6, 0.86; quinidine: 32.0, 0.57; aprindine: 7.6, 0.55; propranolol: 16.6, 0.57; and lidocaine: 20.7, 1.52. The duration of the antifibrillatory effects of 711389-S almost corresponded with those of aprindine and propranolol.