1. A semi-synthetic vitamin B12-deficient diet, based on soya flour, is described. When supplemented with cydnocobalamin the diet appeared to be adequate for growth and reproduction in the rat.2. Compared with their litter-mates on the supplemented diet, rats fed on the deficient diet from weaning showed reduced levels of vitamin B12 activity in the plasma and tissues, but their growth rates were unaffected unless they were bred from mothers that had been given the deficient diet since mating.3. When they were reared on the deficient diet, rats bred from mothers on the deficient diet since mating excreted much more methylmalonic acid in the urine than their litter-mates on the supplemented diet. There was wide variation in the level of excretion, both between different animals and from day to day in the same animal.4. Starvation for more than 16 h caused a marked depression in the amount of methylmalonic acid excreted by rats on the deficient diet.5. Intraperitoneal injection of sodium propionate into deficient animals after starvation for 24 h caused increased excretion of methylmalonic acid during the following 16 h of continued starvation. Isoleucine had a similar but smaller effect.6. Tested in the starved animal, sodium propionate and valine given either by intra- peritoneal injection or by mouth, and isoleucine given intraperitoneally, caused increases in the excretion of methylmalonic acid. In contrast, methionine had no effect and threonine only a slight effect.

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