Action potential duration in the right ventricle is normally shorter than that in the left. We tested the hypothesis that there may be intrinsic differences in the QT and T p-e (an interval from the peak to the end of the T wave) intervals between the left and right chest leads that can be exaggerated by systemic hypertension but attenuated by pulmonary hypertension in humans.Electrocardiograms in the left (V 4 L-V 6 L) and right (V 4 R-V 6 R) chest leads were obtained in 40 healthy individuals, 29 patients with systemic hypertension and left ventricular hypertrophy, and 15 patients with pulmonary hypertension.In healthy individuals, the corrected QT (QTc) and corrected T p-e [T (p-e)c ] intervals were 421 ± 5 and 86 ± 3 milliseconds in V 4 L through V 6 L, respectively, significantly longer than those recorded from V 4 R through V 6 R (383 ± 5 and 62 ± 4 milliseconds, respectively; P < .01). Left ventricular hypertrophy prolonged the QTc interval in V 4 L through V 6 L (456 ± 5 milliseconds), exaggerating the difference in the QTc interval between the left and right chest leads (61 ± 4 vs 40 ± 3 milliseconds in healthy control subjects; P < .01). Left ventricular hypertrophy also resulted in a small but significant increase in the T (p-e)c interval in V 4 L through V 6 L (97 ± 3 vs 86 ± 3 milliseconds in control subjects; P < .05) but exerted no significant effect on the T (p-e)c interval in the right. In contrast, pulmonary hypertension lengthened the QTc interval in the right chest leads, reducing the difference in the QTc interval between the left and right chest leads (3 ± 8 vs 40 ± 3 milliseconds in control subjects; P < .01).There are intrinsic differences in the QT and T p-e intervals between V 4 L-V 6 L and V 4 R-V 6 R that are significantly amplified by systemic hypertension but markedly attenuated by pulmonary hypertension.