Previous studies indicate that sinusoidal endothelial cells bind and internalize hyaluronic acid at much greater rates than do other liver cells. Thus hepatic hyaluronic acid removal rate may be indicative of sinusoidal endothelial cell function. In these studies the uptake of hyaluronic acid (molecular weight 1.3 x lo6) was measured in isolated perfused rat liver under a variety of conditions. Uptake was dependent on hyaluronic acid concentration. At all concentrations tested, the rate of hyaluronic acid uptake stabilized at a steady-state level 2 to 3 min after development of a high rate of apparent uptake. At saturating hyaluronic acid concentration (150 ng * mip1), the steady-state uptake rate was 10.4 & 1.0 pg-gn-' liver wet wt . hr-l, which is as high as or higher than the rates reported for isolated rat liver sinusoidal endothelial cells. The half-maximal rate of uptake was attained at a hyaluronic acid concentration of 80 ng.ml-l. Hyaluronic acid uptake was inhibited by heparin (80%), a competitive ligand for the hyaluronic acid receptor on sinusoidal endothelial cells; 4f3phorbol 12P-0-myristoyl 13a-acetate (25% to 50%), a tumor promoter and activator of protein kinase C; prostaglandin FZa (24% to 52%), an eicosanoid secreted in the liver by Kupffer cells; A23187 (33% to 66%), a Ca2 + ionophore; and Escherichia coli lipopolysaccharide (16% to 43%). Platelet activating factor did not affect hyaluronic acid uptake by the perfused liver. Hyaluronic acid uptake was increased by 50% after a 24-hr fast. Because previous studies indicate that the hepatic sinusoidal endothelial cells are almost exclusively responsible for the hepatic clearance of circulating hyaluronic acid, this study suggests that measurement of hyaluronic acid uptake by the perfused liver may allow assessment of sinusoidal endothelial cell function under the conditions in which the in viuo architecture of the liver remains intact. (HEPATOLOGY 1993;17:266-272.) The importance of functional interrelationship among the various liver cell types is gaining recognition.