Ceruloplasmin, a blue copper oxidase found in plasma, is synthesized in hepatocytes as a single polypeptide chain consisting of a 19 amino acid leader peptide plus 1046 amino acids of mature protein (132 kDa). Holoceruloplasmin is secreted into the plasma with 6-7 atoms of copper bound per molecule. In this study we identified apo- and holoceruloplasmin and examined the mechanism of copper incorporation during ceruloplasmin biosynthesis using the Long-Evans Cinnamon (LEC) rat which does not incorporate copper into newly synthesized ceruloplasmin. We followed the conversion from ceruloplasmin precursor (with little or no carbohydrate) to the larger product (after carbohydrate addition), which occurred in the secretory compartments of hepatocytes, by native gel electrophoresis. We found that copper accumulates in the hepatocellular Golgi apparatus of LEC rats due to a disorder in the process of copper incorporation. The data indicate that copper is incorporated into ceruloplasmin late in the course of its transport through the secretory compartments.