Both resting tachycardia and irregular ventricular rhythm may contribute to impaired cardiac performance in atrial fibrillation (AF). This study assesses the relation between resting heart rate and beat-to-beat changes in left ventricular (LV) ejection and filling in patients with normal and impaired LV systolic function. Beat-to-beat variation in LV outflow and inflow velocity-time integral was measured using pulsed Doppler ultrasound in 39 patients with chronic AF and normal (n=22) or impaired (n=17) LV systolic function. Aortic velocity-time integral variability increased with mean heart rate (p=0.003) even though RR interval variability decreased (p <0.001). Aortic velocity-time integral was more sensitive to the duration of both the preceding (p <0.001) and prepreceding (p <0.001) RR intervals at higher heart rates. These relations were similar for patients with normal and impaired LV systolic function. The sensitivity of the filling velocity-time integral to RR interval variability also increased with heart rate (p <0.001). However, at higher heart rates the filling velocity-time integral (p=0.009) and filling time (p=0.005) were less sensitive to change in RR intervals in patients with impaired LV function. We conclude that beat-to-beat stroke volume variability in AF increases with heart rate. Stroke volume variability was not influenced by LV systolic function.