Monkeys (Macacus rhesus) pre-exposed to mild cold air (10 0C) for 6 mos. and receiving daily 25 mg. of vit. C do not resist intense cold more than the controls kept at room temp. and receiving the same amt. of vit. C. Monkeys pre-exposed to the same mild cold invironment for the same period but receiving at the same time daily 325 mg. of vit. C are far more resistant to intense cold than a) the controls kept at room temp., and b) the previous group also pre-exposed to cold air but receiving daily only 25 mg. of vit. C. Resistance to cold was estimated by the ability of the different groups or individuals to maintain their rectal and intramusc. temps. when exposed suddenly to air of -20[degree]C.