Recent human studies have shown that cold exposure increases lipid oxidation, even when the oxidation of circulating free fatty acid (FFA) is markedly reduced by the ingestion of nicotinic acid, thus seriously questioning the importance of FFA for lipid oxidation in the cold-exposed humans. It was therefore hypothesized that similarly to prolonged exercise, fatty acids from plasma triglycerides (TG) are important energy substrates for oxidation during prolonged cold exposure in man. The goal of this study was to determine the influence of cold exposure on an index of plasma TG utilization, the intravenous fat tolerance test (IVFTT). To evaluate the possibility of a delayed increase in fat tolerance, a second cold exposure and an IVFTT were also performed 24 hours after the first cold exposure. Seven healthy males (fasting, seminude) were subjected to an IVFTT (1 mL/kg 10% Intralipid) on three occasions while resting for 160 minutes: (1) at 29°C, (2) in the cold (10°C, 1 m/s wind), and (3) at 10°C 24 hours after the first cold test. One week separated the warm test from the cold tests. Cold exposure reduced mean body temperature by 3.4 ± 0.1°C and increased energy expenditure 2.5 times in comparison to warm values ( P < .01). It also increased fat oxidation by 70% ( P < .05) and plasma glycerol levels ( P < .05), but did not alter fat tolerance. Although the second cold test entailed essentially the same changes in body temperatures and heat production as the first one, the second cold test was accompanied by a further increase in fat utilization (132% above warm values, P < .01), slightly higher plasma glycerol levels, and an unchanged fat tolerance. The results of the present study demonstrate that cold exposure in humans significantly increases the oxidation of lipid, and that plasma TG do not appear to be an important energy substrate in the cold, even when lipid metabolism is further increased by the second cold test. It is suggested that white adipose tissue TG and intramuscular TG, not plasma TG, are the preferred sources of fatty acids for oxidation in cold-exposed humans.