To assess determinants of left ventricular filling time at rest simultaneous phonocardiograms, M mode echocardiograms, and indirect carotid pulse traces were performed on 50 patients with atrial fibrillation. Thus the effect of random variation in RR interval in the absence of atrial contraction on the inter-relation between systolic and diastolic events could be seen. Left ventricular filling time was closely related to RR interval and affected by it to a much greater extent than ejection time, so that filling time became very short when heart rate was fast. Two mechanisms were, however, identified whereby filling time was preserved: (a) prolongation of filling during the pre-ejection period of the following cycle, so that when the RR interval was less than 400 ms all of ventricular filling frequently occurred at this time; and (b) abbreviation of the QA2 interval of the succeeding beat with reduced filling, so that mitral valve opening occurred early. Changes in the duration of isovolumic contraction or relaxation time were not involved, neither could alteration in left ventricular contractile state resulting from variation in the interval between beats be shown to affect time at rest. The striking sensitivity of left ventricular filling time to rapid heart rate in atrial fibrillation suggests that it may be a major factor limiting cardiac performance. Diastolic as well as systolic events should, therefore, be taken into account when the effects of drugs or other physiological manoeuvres are studied in these patients.