The effect of mitral valve closure on left ventricular filling time and its relation to the onset of systole were assessed from mitral valve echocardiograms and simultaneous apex cardiograms in 21 normal subjects, 11 patients with left bundle branch block, and 19 patients with VDD pacemakers programmed for atrioventricular intervals of 50, 150, and 250 ms. The interval between the electrocardiograph Q wave and the apex cardiogram upstroke was similar in normal subjects and patients with left bundle branch block, but was significantly longer in patients with VDD pacemakers at all atrioventricular intervals. Similarly there was little difference in the time interval between the Q wave and mitral valve closure in normal individuals and patients with left bundle branch block but this was considerably delayed in VDD pacemaker patients with the atrioventricular interval set at 50 ms. With increasing atrioventricular intervals the mitral valve closed significantly earlier, whereas the onset of left ventricular systole and the timing of mitral valve opening remained unchanged. Thus as a result of earlier mitral valve closure left ventricular filling time decreased progressively as the atrioventricular interval was increased. Since the onset of left ventricular systole, with respect to left ventricular stimulation, is considerably delayed in VDD pacemaker patients a short atrioventricular interval is required in these patients to maintain the normal time relations between atrial and ventricular contraction and hence maximise left ventricular filling.