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1) reported that "choline oxidase" activity was low in the livers of young rats. T2.lese investigators did not distinguish between two of the enzymes involved in choline oxidation, choline dehydrogenase (CD) and betaine aldehyde dehydrogenase (BAD), nor was a study made of diphosphopyridine nucleotide (DPN) , the coenzyme required by BAD and shown to stimulate CD activity( 2,3). The work reported here was undertaken to determine which of these components was responsible for the low rate of choline oxidation by livers of young rats.Methods. Rats of the Sprague-Dawley strain maintained on a dog chow ration were used in this study. The DPN content, choline dehydrogenase and betaine aldehyde dehydrogenase activities in the livers of these rats were determined at specified times after birth. Manometric technics were used to measure choline dehydrogenase and betaine aldehyde dehydrogenase activities. For the determination of choline dehydrogenase the method of Strength, Christensen and Daniel (3) was used with the addition of 2 pmol of DPN per flask. Betaine aldehyde dehydrogenase activity was measured by the method of Christensen(4), as follows: To the main compartment of the Warburg flask were added 1 ml of 0.1 M barbiturate buffer, p H 8.4 containing 0.1% nicotinamid-e; 3 pmol DPN; 18 pmol methylene blue; 14 pmol betaine aldehyde chloride; and enough water to bring the total volume to 2.6 ml. The center well con-