1. The metabolism of radioactive vitamin D3 has been studied in individuals low or deficient in vitamin D (group I) and in vitamin D treated subjects (group II). 2. In group I there was a smaller serum pool of vitamin D, turning over more rapidly than in group II. The principal metabolite, peak IV, appeared more rapidly in the serum of group I; the level of radioactivity attained in this and in the more polar metabolites, peak V and VI, was also higher than in group II. Peak VI was the major radioactive component in serum after 100 days. 3. Vitamin D treatment of individuals in group I converted the pattern of metabolism of radioactive vitamin D3 to that characteristic of group II. This effect was observed in healthy individuals and in patients with vitamin D deficiency or with chronic renal failure. 4. The metabolic disposal of vitamin D entering the body appears to be determined by the state of vitamin D nutrition in the individual. Reported changes of vitamin D metabolism in diseases such as renal failure could be determined by the nutritional state of the patients studied rather than by the primary disease.