Background Previous studies from our laboratory have shown pronounced augmentation of the force-frequency relation on myocardial contraction during exercise, but the influence of this effect on diastole has not been investigated.Methods and Results Accordingly, the effect of changing heart rate on left ventricular (LV) relaxation, filling dynamics, and pressure-volume relations during exercise was studied in eight conscious dogs. The exercise heart rate was slowed from 208+-21 (SD) to 163+-11 beats per minute by injection of a specific sinus node inhibitor (UL-FS 49, or zatebradine, 0.6 mg/kg) during continuous exercise. Heart rate was then abruptly restored to the predrug level by atrial pacing during continued exercise. LV volume was calculated by use of implanted ultrasonic crystals, and LV pressure was determined with an implanted micromanometer. Comparing conditions after pacing back to a heart rate of 210 beats per minute with those obtained when the heart rate was slowed by atrial pacing, LV dP/dtmaxwas increased by 27% at the higher rate (P<.01), despite a marked decrease in LV end-diastolic pressure (24+-4 versus 10+-5 mm Hg, P<.01), and the time constant of isovolumic LV pressure decay (tau) was significantly shortened (19+-5 versus 14+-4 milliseconds, P<.01). The peak rapid filling rate in early diastole (PFR) was not significantly changed by increasing the heart rate, since it was maintained at the slower rate. During exercise, at the slowed heart rate the early portion of the diastolic pressure-volume curve was significantly shifted upward and to the right compared with that at the physiological heart rate, but the late portion of the curve was unchanged.Conclusions These data indicate that the negative inotropic effect of the force-frequency relation when heart rate was slowed during exercise caused pronounced impairment of LV relaxation and early filling dynamics. Conversely, an important component of the pronounced improvement of diastolic ventricular function during normal exercise was shown to result from exercise-induced enhancement of the positive inotropic effects of the force-frequency relation on myocardial contraction and relaxation. (Circulation. 1994;89:2361-2368.)