To determine the electrophysiologic effects of stellate ganglion (SG) block on the human heart, the two SGs were anesthetized separately, with a 24-hour interval between the two procedures, in 13 patients with episodes of supraventricular tachycardia (six had Kent bundles). Left SG block caused: (1) a lengthening of the AH interval, measured at fixed atrial rates of 10 +/- 12 msec (p less than 0.01); (2) a marked depression of the VA conduction in six of the seven patients with measurable VA interval (in two patients it produced complete VA block); (3) a slowing of 20 to 40 msec of the cycle of an electrically induced reciprocating tachycardia; and (4) failure to modify the QT interval duration. In contrast, right SG block produced asymmetric or opposite changes and prolonged the QT interval (7.6 +/- 8.8 msec, p less than 0.05). Atrial and ventricular refractoriness was not significantly altered by SG block. Retrograde effective refractory period of the Kent bundle changed 20 to 60 msec after unilateral SG blockade. Thus, this study suggests that the human conduction system and the Kent bundles receive an appreciable sympathetic influence from the SG. Like experimental studies, we also found an asymmetric response to unilateral SG block and a dominance, in most of our patients, of the left SG. The influence on myocardial refractoriness was less apparent.