Purified fractions of three major rat high-density lipoproteins (HDL) and one rat very low-density lipoprotein (VLDL) were isolated by Sephadex gel chromatography or preparative sodium dodecyl sulfate gel electrophoresis. These proteins were characterized by amino acid analysis, end-group analysis, molecular-weight determination, polyacrylamide gel electrophoresis, and circular dichroism. One of these rat proteins, of molecular weight 27 000, appears to be homologous with the human A-I protein. However, rat HDL possesses two additional major components not reported in human HDL - an arginine-rich protein of molecular weight 35 000 and a protein of molecular weight 46 000. The arginine-rich protein of the rat is similar in size and amino acid analysis to the arginine-rich protein reported in human VLDL. A major component of rat VLDL of 35 000 molecular weight appears similar or identical to the arginine-rich protein in rat HDL by every criterion employed for their characterization.