Vit. B12 or liver extract administered to force-fed rats on constant food intake induced neither wt. gain nor N retention. When vit. B12 was administered to force-fed rats on constant food intake, made hyperthyroid by injn. of thyroxine, the wt. loss was identical with that of hyperthyroid animals not receiving vit. B12. The N loss resulting from the catabolic action of thyroxine, however, was significantly smaller in thyroxine-treated rats receiving B12 than in the thyroxine-treated controls. Liver extract had a similar action, but the values obtained were statistically not significant. These results suggest that in hyperthyroid rats vit. B12 spares protein at the expense of other body constituents.