Peak rapid filling rate (PRFR) is often used clinically as an index of left ventricular relaxation, i.e., of early diastolic function. This study tests the hypothesis that early filling rate is a function of the atrioventricular pressure difference and hence is influenced by the left atrial pressure as well as by the rate of left ventricular relaxation. As indexes, we chose the left atrial pressure at the atrioventricular pressure crossover (PCO), and the time constant (T) of an assumed exponential decline in left ventricular pressure. We accurately determined the magnitude and timing of filling parameters in conscious dogs by direct measurement of phasic mitral flow (electromagnetically) and high-fidelity chamber pressures. To obtain a diverse hemodynamic data base, loading conditions were changed by infusions of volume and angiotensin LI. The latter was administered to produce a change in left ventricular pressure of less than 35% (A-1) or a change in peak left ventricular pressure of greater than 35% (A-2). PRFR increased with volume loading, was unchanged with A-1, and was decreased with A-2; T and PCO increased in all three groups (p < .005 for all changes). PRFR correlated strongly with the diastolic atrioventricular pressure difference at the time of PRFR (r = .899, p < .001) and weakly with both T (r = .369, p < .01) and PCO (r = .601, p < .001). The correlation improved significantly when T and PCO were both included in the multivariate regression (r = .797, p < .0001). PRFR is thus determined by both the left atrial pressure and the left ventricular relaxation rate and should be used with caution as an index of left ventricular diastolic function.Circulation 74, No. 1, 187-196, 1986. RELAXATION ABNORMALITIES are one of the earliest manifestations of cardiac dysfunction and frequently precede systolic dysfunction in many disease states.'' Early filling function has been evaluated in a variety of diseases, e.g., coronary artery disease, hypertrophic cardiomyopathy, hypertensive heart disease, aortic valve disease, and congestive cardiomyop-