Some observations on the vitamin B12 content of rat's milk.

I. Vitamin B,, activity in the milk of rats on various diets was measured by microbiological assay at intervals throughout lactation. The concentration of sodium and calcium in the milk was also determined. 2. At all stages of lactation the milk of rats fed on commercial rat cake contained much more vitamin Bla activity than that of rats receiving a semi-synthetic diet containing at least as much vitamin B,, as the rat cake. The milk of the latter group of animals contained more vitamin B,, than that from rats fed on the same diet without added cyanocobalamin. The levels of sodium and calcium were much less variable than those of vitamin Bla and the changes did not appear to be related to differences in diet, stage of lactation or litter size. Weanling rats fed on a vitamin B,,-deficient diet were found to grow at normal rates unless they were bred from mothers that had received the deficient diet since mating. Also, the bodies of the offspring of such mothers contained less vitamin B,, than those of mothers receiving either a vitamin B,,-supplemented diet or rat cake (Williams, Spray, Newman & O'Brien, 1969). It had been shown previously that during the first few days of life, rats absorb about 90 yo of an oral dose of cyanocobala-min mixed with rat's milk, compared with only about 50 % later (Williams & Spray, 1968). In view of these observations it seemed to be of interest to study the vitamin B,, content of the milk of rats receiving various diets. Animals and their management. Female albino rats of the Wistar strain received ad lib. a basal diet deficient in vitamin BIZ. choline dihydrogen citrate and mineral salts and vitamins (Williams et al. 1969). For some animals the diet was supplemented with 1 5 pg cyanocobalamin/kg diet. The rats were housed and managed as described previously (Williams et al. 1969). Rats receiving rat cake (modified diet 41 B; Herbert C. Styles (Bewdley) Ltd) were kept in ordinary rat cages. The animals were mated and those that became pregnant were transferred to individual breeding boxes, where they remained until the young were weaned. Where necessary, surplus young were removed or young of the same age from other mothers on the same diet were added, to maintain the litters at the desired size (Table I). Collection of milk. After being separated from their young for 18 h, …