The aim of this study was to examine the effect of ventricular fibrillation and a subsequent defibrillation shock on ventricular excitability and refractoriness in human beings. We studied 16 consecutive patients with implantable cardioverter-defibrillators undergoing follow-up studies. The pre- and post-shock pacing threshold, ventricular effective refractory period, monophasic action potential duration, and serum catecholamine levels were measured. Compared with the baseline state, immediately after ventricular fibrillation, and a successful defibrillation shock: (1) the ventricular effective refractory period decreased from 251 ± 24 ms to 222 ± 30 ms (p < 0.01), (2) the monophasic action potential duration decreased from 210 ± 16 ms to 179 ± 23 ms (P < 0.01) at 50% repolarization and from 274 ± 24 ms to 240 ± 26 ms (P < 0.01) at 90% repolarization, (3) the pacing threshold was not significantly altered and, (4) serum levels of epinephrine and norepinephrine were elevated. These results show that although ventricular fibrillation and subsequent defibrillation had no effect on the ventricular pacing threshold in human beings, it was associated with a decrease in post-shock monophasic action potential duration and ventricular effective refractory period, contrary to some previously reported findings.