The existence of thermoregulatory nonshivering thermogenesis, with special reference to lipid metabolism, was investigated in men. Acute cold exposure(10• Ž, 60 min) produced a marked increase in heat production, with concomitant elevation of plasma free fatty acid (FFA) level, modest increase of ketone body concentration and lowered respiratory quotient(RQ). The correlation of heat production to plasma FFA levels was significantly positive; that is, subjects with higher heat production showed higher plasma FFA levels. Moreover, correlation of either heat production or plasma FFA levels to RQ was significantly negative, respectively. On the other hand, exposure to cold after an administration of nicotinic acid, which has a suppressive effect on FFA mobilization from adipose tissue, resulted in less cold-elevated heat production, a significant fall of plasma FFA and ketone body concentrations, and no change in RQ Although no visible or only slight shivering was observed in control cold exposure study, greater shivering occurred in the nicotinic acid cold exposure study.