The effects of the activation sequence on ventricular repolarization and its spatial gradient were examined in anesthetized open-chest dogs. Unipolar and bipolar electrograms were recorded from 47 epicardial sites on the anterior left ventricular wall using a mapping electrode. The local QT interval (QT) and the activation time (AT) at each site were measured on the unipolar and bipolar electrograms, respectively. The QT index (QTI) was defined as the QT minus AT interval, and was used as a measure of local repolarization. QTI was longer at each site during propagation that was longitudinal (L) (219+/-21 ms) than during propagation transverse (T) (202+/-22 ms, p<0.001) to the epicardial fiber orientation or during atrial pacing (165+/-20 ms, p<0.001). During L-propagation, the QTI shortened as a function of the distance from the stimulus. The spatial gradient was steeper during T-propagation (p<0.05). Monophasic action potentials (MAP) were also recorded simultaneously at 4 epicardial sites. The MAP duration during ventricular pacing was longer than during atrial pacing at sites within 1.5 cm of the pacing site. This difference disappeared at more distant sites and was attenuated by a simultaneous stimulus from a site symmetrically aligned along the fiber. These findings indicate that anisotropic conduction prolongs ventricular repolarization and increases its spatial gradient in the intact heart. An electrotonic downstream effect appears to be the cause.