To determine the effect of heart rate alterations on diastolic timing intervals and filling parameters, 10 normal patients were paced from the right atrium at 30 and 50 beats/min above their baseline rates. M-mode echocardiograms of the aortic valve, mitral valve and left ventricle were obtained and digitized at baseline and with each pacing rate. With increased atrial pacing, left ventricular systolic time became an increasingly greater proportion of cycle length while the diastolic filling period occupied a lesser proportion of the cycle length. The time to peak filling rate and the rapid filling period occupied a greater proportion of the diastolic filling period. The peak filling rate increased progressively with increased atrial pacing (baseline 128 +/- 19 mm/s, first paced rate 146 +/- 27 mm/s, p less than 0.05 vs baseline; second paced rate 167 +/- 23 mm/s, p less than 0.01 vs baseline and first paced rate). The early diastolic filling fraction and rapid filling fraction also increased with pacing. Increasing the heart rate resulted in an alteration of the time course of diastolic filling and extent of diastolic filling during the rapid filling period. Interventions that improve diastolic filling and increase heart rate may in part be due to heart rate changes.