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Results are presented which indicate that in guinea pigs cyanocobalamin undergoes conversion to hydroxocobalamin, in vivo and in vitro, at the approximate rate of 0.1-0.4 mµg./day/Gm. of liver. In boiled liver, no conversion was found. The radioactivity excreted in human urine the first 12 hours after a therapeutic cyanocobalamin dose, on the other, is still cyanocobalamin. It is suggested that demonstrated metabolic differences between therapeutically used cyanocobalamin and hydroxocobalamin are explained, in part, by the time needed to convert cyanocobalamin to hydroxocobalamin.

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