Effects of continuous cold stress on daily patterns of oxygen consumption (Vo2) and rectal temperature were studied in five men. Cold stress consisted in living continuously in a chamber at 60°F for 14 days, wearing only shorts and with minimal physical activity. Resting Vo2 and rectal temperatures were measured at 8 a.m., 12 m, 4 p.m. and 8 p.m. The cold period was preceded and followed by 2 weeks at 80°F. Activity and dietary composition were the same for all periods. Resting Vo2 during warm periods exhibited gradual and characteristic increases during the day. This pattern was also found during cold exposure, but at a higher level; Vo2 in the cold was 20% higher at 8 a.m., 16% at 12 m, 16% at 4 p.m. and 11% higher at 8 p.m. than at corresponding hours during control periods. Basal metabolic rate did not change throughout the experiment. Rectal temperatures at noon, 4 p.m. and 8 p.m. throughout the cold period did not differ from those at 80°F. Rectal temperatures at 8 a.m. were significantly higher in the cold than at 80°F. The results indicate that rectal temperature was well maintained during cold exposure and oxygen consumption appeared to respond in such a fashion as to subserve this maintenance. Submitted on November 15, 1956