Abstract
The effects of crystalline vit. B12 on thiouracil action was detd. in immature female rats for a 30-day period. The vit. completely counteracted the growth-inhibiting action of thiouracil, and this was accompanied by a considerable increase in food consumption. Although vit. B12 decreased the thyroid hypertrophy introduced by thiouracil, the uptake of radioactive iodine (I131) by the thyroids was even less than in the rats which received thiouracil only. It is suggested that vit. B12 may be able to induce normal growth in hypothyroid rats.