Electrophysiologic effects of intravenous E-4031 and MS-551, novel class III antiarrhythmic agents, were evaluated in 5 and 6 patient with ventricular tachyarrhythmia, respectively. Six patients had sustained ventricular tachycardia (VT) and 5 had ventricular fibrillation (VF). Electrophysiologic study was performed before and after administration of E-4031 and MS-551 [E-4031; loading infusion 9 micrograms/kg for 5 min + 0.15 microgram/kg/min, MS-551; loading infusion 0.3 mg/kg for 5 min + 0.01 mg/kg/min]. The QT intervals were significantly prolonged after administration of E-4031 and MS-551 from 409 +/- 37 to 455 +/- 49 msec (11%), and from 359 +/- 52 to 411 +/- 63 msec (14%), respectively. The QTc intervals were significantly prolonged from 457 +/- 17 to 494 +/- 24 msec (8%), and from 410 +/- 36 to 452 +/- 47 (10%), respectively. There were no significant differences in the QT and QTc intervals between these two agents. The right ventricular effective refractory period (VERP) with E-4031 was prolonged at 600 (from 244 +/- 27 to 270 +/- 31 msec, 11 +/- 2%), 400 (from 222 +/- 23 to 242 +/- 24 msec, 9 +/- 3%), and 300 msec (from 206 +/- 19 to 218 +/- 25 msec, 6 +/- 4%), and those with MS-551 were prolonged at 600 (from 240 +/- 23 to 268 +/- 23 msec, 12 +/- 2%), 400 (from 225 +/- 22 to 250 +/- 24 msec, 11 +/- 4%), and 300 msec (from 213 +/- 14 to 228 +/- 18 msec, 7 +/- 4%). Both E-4031 and MS-551 prolonged VERP in a "reverse" use-dependent manner without changing the conduction velocity. E-4031 prevented the induction of VT in one patient. MS-551 prevented the induction of VT and VF in one patient each. Further evaluation of these selective class III agents may be needed to determine if higher doses are required to achieve the pharmacological effects in patients with ventricular tachyarrhythmias.