The vit. B12 content of the livers, spleens, hearts and kidneys of 28-day-old rats was determined. The mothers received the synthetic basal diet or the basal diet supplemented with a vit. B12 concentrate from different sources. The amt. of vit. B12 in the liver, spleen, and heart was approx. doubled when a source of vit. B12 was added to the diet of the mothers. The diet of the mother had no important effect on the vit. B12 content of the kidney. The kidney and spleen contained slightly more vit. B12 on the avg. than the liver. The amt. of vit. B12 in the heart was small in comparison with other tissues.