The slope of the regression equation for left ventricular ejection time (LVET) vs heart rate (HR) is the appropriate factor for correcting LVET for HR. Because the regression relationship varies under different conditions, we determined the LVET-HR equations for subjects (1) seated at rest on a bicycle ergometer, and (2) during uninterrupted bicycle exercise. In 18 normally active male volunteers, ages 22-37, HR and LVET were measured under the two conditions and the regression relationship for LVET on HR determined for each. Regression equations are as follows: (1) LVET = 379-1.8 HR +/- 11.0 for subjects seated at rest on a bicycle ergometer, and (2) LVET = 371-1.2 HR +/- 13.9 for subjects performing upright bicycle exercise. The slope factors (1.8 and 1.2) differed significantly (p less than 0.01). The data indicate that considerable error can result from arbitrarily applying to exercising or resting subjects a correction factor which does not fit the conditions of the data to be corrected.