The effect on ventricular rate of intracardiac shocks for atrial fibrillation was studied in 13 patients receiving 95 shocks. Shocks were synchronized to the R wave and were delivered after R-R intervals >500 msec, with increasing strength (20 to 400 V). In 10 patients, conversion to sinus rhythm was achieved in this way. Noneffective shocks increased the mean first postshock R-R interval (compared with 20 V as baseline), for shocks ≥140 V. The R-R prolongation correlated with the shock level ( r = 0.936, p < 0.01). Subsequent R-R intervals were comparable to baseline. A similar trend toward longer first postshock intervals was observed for effective shocks. However, the maximal first R-R interval after noneffective shocks had no relation to the voltage. The number of pauses >1500 msec tended to increase with voltage (noneffective shocks). Pauses >2500 msec were exceptional (4 of 85 noneffective shocks). No symptomatic bradycardia occurred. In subsequent intervals (cycles 2 through 10) no pauses >2500 msec were noted. It is concluded that atrial defibrillation attempts between the right atrium and coronary sinus prolong R-R intervals, in relation to administered energy, but without the need for backup pacing. (Am Heart J 1997;133:101-7.)