The effectiveness of a catheter having one electrode in the right ventricle and the other electrode in the superior vena cava for reversing fibrillation in dogs was studied with four types of truncated exponential stimuli in 576 episodes. A catheter was inserted through a jugular vein and its tip positioned under fluoroscopic monitoring in six large animals. On the day of catheter insertion and at one and two weeks, each animal was subjected to eight defibrillation attempts with each of four wave forms. Wave forms having an initial current of 7 amp, a final current of 3.5 amp, and time constants of 10 and 20 msec were 50% and 81% effective, respectively. Wave forms having an initial current of 10 amp, a final current of 5 amp, and time constants of 5 and 10 msec were 60% and 98% effective, respectively.