During 7 hours of comfortably warm sleep, average metabolic rates of 11 Indian and 7 white men were alike. Since the Indians were 15% lighter their metabolic rates (MR's) referred to weight were greater. During a night at 0° with insufficient covering, MR's rose to 129% and 132% in the two groups. Cold caused equal myographic records of shivering (15% and 13% of records) and gross muscular movement appeared in 6.5% of the records for each group. Encephalograms showed that Indians slept more (51%) than whites (31%). Shivering was recorded in Indians and white men during encephalographic indications of sleep. Rectal temperatures of Indians declined about 0.5°. During cold nights skin on the bodies of all subjects cooled 3°–5° and about 15° on the feet. All subjects were disagreeably cold, but their cold sensations stimulated metabolic heat production only half as much as would be necessary to maintain fairly comfortable warmth.