Copper added to a standard diet often effects a moderate increase in hemoglobin production in anemia due to blood loss. The copper response is quite irregular in contrast to the iron response. In these dogs there is no lack of copper held in reserve stores (liver and spleen) so the reaction is not related to an actual deficiency of the element. An effect upon enzyme complexes related to globin and hemoglobin production is to be considered. Cobalt under similar conditions causes no stimulus to hemoglobin production, rather an inhibitory effect when more than minimal doses are given. The claim that cobalt causes a polycythemia in dogs receives no support from our experiments.