The sympathetic nervous system is thought to play a key role in genesis and maintenance of ventricular arrhythmias. The myocardial effect of sympathetic stimulation on myocardial repolarization in humans is poorly understood. The purpose of this study was to evaluate the effects of direct and reflex sympathetic stimulation on ventricular repolarization in patients with postinfarct cardiomyopathy (ICM). The effects of direct sympathetic stimulation were assessed using isoproterenol, while those of reflex sympathetic stimulation were assessed with nitroprusside infusion in ICM patients (n = 5). Five patients without cardiomyopathy were also studied. Local repolarization was measured from intracardiac electrograms that were used to calculate the activation recovery interval (ARI), a surrogate of action potential duration. Isoproterenol significantly increased heterogeneity in repolarization in patients with ICM; the decrease in ARI from baseline was 72.9 ± 9.1 ms in more viable regions, 64.5 ± 8.9 ms in the scar, and 54.9 ± 9.1 ms in border zones (P = 0.0002 and 0.014 comparing normal and scar to border zones, respectively). In response to nitroprusside, the ARI at the border zones decreased significantly more than either scar or surrounding viable myocardium, which showed an increase in ARI (P = 0.014 and 0.08 comparing normal tissue and scar to border zones, respectively). Furthermore, isoproterenol increased ARI dispersion by 70%, while nitroprusside increased ARI dispersion by 230% when ICM patients were compared to those with structurally normal hearts (P = 0.0015 and P < 0.001, respectively). In humans, both direct and reflex sympathetic stimulations increase regional differences in repolarization. The normal tissue surrounding the scar appears denervated. Dispersion of ARI in response to sympathetic stimulation is significantly increased in patients with ICM.