Anker and Bloch (1) have shown that, when deuterium-labeled cholestenone (A4-cholestene-3-one) is fed to rats, the sterol mixture from the carcass, blood, and liver becomes labeled, most of the deuterium being associated with cholestanol. However, a “small but significant” deuterium concentration was reported associated with cholesterol, after purification through the dibromide. While t, heir observations exclude cholestenone as a major precursor of cholesterol, the possible conversion of a A4-ketone to a So-hydroxy-A6 compound is of interest. We have therefore determined the relative amounts of activity present in the purified cholestanol and cholesterol in rat liver after injection of cholestenone-4-Cl4 into the portal vein.