The apoproteins of serum lipoproteins and of lipoproteins present in a nonrecirculating perfusate of rat liver were compared by immunochemical, gel electrophoretic, and solubility techniques. Serum and perfusate very low density lipoprotein apoprotein composition were not different. No evidence for the presence of a lipoprotein resembling serum low density lipoprotein was obtained. However, the apoprotein composition of circulatory high density lipoprotein was quantitatively different from the secretory product in the density 1.06-1.21 range. As measured by stained sodium dodecyl sulfate gel electrophoretic patterns, the arginine-rich protein was the major secretory apoprotein while the A-I protein was the major apoprotein in circulating high density lipoprotein. A very similar pattern was seen in perfusates of orotic acid-fatty livers. It was concluded that although the liver secrets lipoproteins in the high density class, circulatory high density lipoprotein is largely a product of catabolic processes.