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Rats were exposed to 4 or 25 degrees C for 6, 12, and 24 h or for longer periods up to 2 wk, then eviscerated. The hourly changes in plasma alpha-amino nitrogen were followed for 4 h after operation, which because of functional removal of the liver allows amino acids released from the extrahepatic tissues to accumulate in the blood. Although cold exposure of tissues to accumulate in the blood. Although cold exposure of 24-h-fasted rats for 6 h before eviscerated resulted in an increased rate of rise of plasma alpha-amino nitrogen postoperatively, the rise was not significantly greater than warm controls until after 12 h of exposure. Thereafter, similar results were obtained whether cold was 12, 24, or 48 h or 14 days duration. Fasting in the cold produced an immediate sharp rise after evisceration so that the concentration was 2 times the control value in 2 h. By contrast, animals allowed to eat adlibitum showed the rise after evisceration but it was progressively smaller in amount as the exposure extended beyond the 1st day. After 5 days there were no longer significant differences from control values. The reduction in rate of rise in plasma alpha-amino nitrogen coincided with cold-induced increases in food intake. The findings support the view that protein metabolism of muscle, the predominant extrahepatic tissue, may participate in thermogenesis when other sources of energy in the body have been depleted.