The relationship between the duration of electrical systole (QT) and heart rate (HR) and the relationship between the QT interval and total electromechanical systole (QS2) were studied in the resting state in 200 patients (100 males and 100 females) without evidence of cardiovascular disease. A linear relationship was found between the QT and HR in males and females (males, QT = 521 msec - 2.0 HR, r = .91; females, QT = 511 msec - 1.8 HR, r = .90). In 20 male and 20 female subjects, the relationship between QT and QS2 was studied. The QT was slightly shorter but paralleled the QS2 (males QT = 529 msec - 2.1 HR, QS2 = 541 msec - 2.2 HR; females QT = 511 msec - 1.9 HR, QS2 = 540 msec - 2.0 HR). Thus, over the physiologic range of resting HR, a linear relationship exists between QT and HR. The QT interval is slightly shorter but parallels the QS2 in patients without heart disease. These linear relationships permit a direct comparison of the duration of electrical and mechanical systole.