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Vitamin D in all body tissues was radio-labeled by supplementing completely vitamin D-deficient weanling rats with oral vitamin D3-4-14C for 2 wk. All vitamin D was then withheld, and radioactivity and vitamin D content in a variety of organs and tissues were measured. Adipose tissue was found to contain by far the greatest quantity of radioactivity throughout the 3 month experimental period. Immediately after supplementation, half of the total radioactivity in adipose tissue corresponded to unaltered vitamin D3, and the other half to polar metabolites and esters of vitamin D3 and unidentified peak II. 1 month later there was approximately the same proportion but a decrease in the total quantity of each form. We conclude that adipose tissue is the major storage site for vitamin D3 in its several forms. Unaltered vitamin D3 was the principal storage form observed and presumably a source available for conversion to other metabolites during deprivation.