The left atrium (LA) stores part of the right ventricular stroke volume during ventricular systole for passive release into the left ventricle (LV) during ventricular early diastole and active release during ventricular late diastole (passive and active contraction). The rest of the right ventricular stroke volume flows through the LA from the pulmonary vascular bed to the LV (LA conduit function). The factors affecting these LA functions were evaluated during ventricular diastole by left atrial angiography and left ventriculography in 21 patients with normal LV ejection fraction (0.53-0.76), but without coronary artery disease and valvular heart disease, aged 40-70 years. LV stroke volume was calculated from left ventriculograms. The maximal LA volume, minimal LA volume, and LA volume at the beginning of the left atrial active contraction were calculated from the left atrial angiograms, and further corrected by a regression equation (true volume = 0.91 x calculated volume -1.1). The LA reservoir volume, LA passive contraction volume, LA active contraction volume, and LA conduit volume were obtained. LA conduit volume correlated significantly with LV stroke volume (r = 0.97), but not with maximal LA volume and LA reservoir volume. LA active contraction volume correlated significantly with LA reservoir volume, LA volume at the beginning of LA active contraction, and maximal LA volume (r = 0.85, 0.60, 0.54, respectively). LA passive contraction volume did not correlate with any factor, and was grossly independent of LV stroke volume. LA conduit volume may be associated with LV diastolic function, and LA active contraction volume appears to increase with increased LA volume.