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Tissue vitamin B12 levels were determined in the blood, liver, kidneys, gastrointestinal tract and uteri and contents in pregnant and nonpregnant female rats following oral administration of three doses of radioactive vitamin B12 (Co60). Differences in radioactivity of tissues of animals administered Co60 vitamin B12 in water and in d-sorbitol at different stages of pregnancy were noted. d-Sorbitol administration significantly enhanced vitamin B12 absorption in both pregnant and nonpregnant rats. Gestation, but not d-sorbitol, causes an alteration of the tissue distribution pattern of the vitamin. As fetal growth progressed the fetuses obtained a large proportion of the absorbed B12 which would normally have gone to the maternal tissues. During growth of the total fetal mass from 3 to 30 gm the amount of B12 retained per gram of tissue remained fairly constant. The amount of B12 taken up by the fetuses was to a great degree controlled by the amount of the vitamin absorbed by or stored in the mother during gestation.

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