Radioactive leucine was administered to male Sprague-Dawley rats in order to measure the rate of release and decay of newly synthesized apoceruloplasmin and holoceruloplasmin in copper-deficient and normocupremic rats, respectively. Anticeruloplasmin antisera were used to isolate the protein from rat plasma. In copper-deficient rats the release of apoceruloplasmin into the circulation proceeded at the same rate as that of the copper-containing protein in normocupremic rats, although the concentration of apoceruloplasmin in copper-deficient rats was only 25% that of normal. This reduction was accounted for by the more rapid disappearance of apoceruloplasmin from the circulation. When copper was administered to copper-deficient rats no increase in the rate of release of either newly synthesized ceruloplasmin or preformed ceruloplasmin was observed, although the ceruloplasmin oxidase concentration rose over 8-fold by 8 hours. This rise was accounted for by the enhanced stability of the copper-containing protein. When 64Cu was administered to copper-deficient rats none of it was bound to circulating apoceruloplasmin, indicating that copper bound to the protein before it was released into the circulation.