To investigate thermoregulation independent of muscular contractions in adult man during a short period of exposure to cold, five examinations were carried out on four individuals who were unconscious due to persistent brain damage and who were totally curarized during the exposure. An increase was found in total body oxygen consumption of 0.043 mmol . kg-1 (range 0.038-0.049) or 25%, in plasma noradrenaline of 5.05 nmol . 1-1 (range: 2.48-6.73) or 132%, and in plasma non-esterified fatty acids of 0.6 mmol . -1 (range: 0.049-0.095) or 44%. No changes occurred in plasma adrenaline or plasma glucose. Splanchnic oxygen consumption increased and contributed significantly to that of the total body in only one of the four cases. These results indicate that regulatory nonshivering thermogenesis exits in adult man not acclimatized to cold, that it is mediated by noradrenaline, that the major fuel substrate for the increased energy demand is the triglycerides from the fat stores, and that the splanchnic area contributes only irregularly and to a small degree to this part of thermogenesis.