To clarify the source of increased carbohydrate oxidation during cold stress, six males rested for 3 h at 29 and 10 degrees C dressed only in shorts. After priming the blood glucose and bicarbonate pools, [U-13C6]glucose was infused for 3 h in each condition to determine the plasma glucose rate of appearance (Ra) or turnover under relative steady-state conditions. Plasma enrichment (mol %excess) was determined by selective ion-monitoring gas chromatography-mass spectrometry. Cold exposure decreased rectal temperature and mean skin temperature and increased heat debt, metabolic rate, and whole body lipid and carbohydrate oxidation (CHOox) compared with the same subjects at thermal neutrality (P < 0.05). Cold exposure significantly increased Ra from 13.18 +/- 0.70 to 16.22 +/- 0.43 mumol.kg-1.min-1 (P < 0.05). Plasma glucose clearance was elevated commensurately by the cold (from 2.68 +/- 0.16 to 3.55 +/- 0.14 ml.kg-1.min-1; P < 0.05). If we assume that Ra is completely oxidized (thus equivalent to maximum rates of plasma glucose oxidation) [J. A. Romijn, E. F. Coyle, L. S. Sidossis, A. Gastaldelli, J. F. Horowitz, E. Endert, and R. R. Wolfe. Am. J. Physiol. 265 (Endocrinol. Metab. 28): E380-E391, 1993], the minimum rates of glycogen and lactate oxidation in the cold would be the difference between CHOox and glucose oxidation (approximately 14.0 +/- 3.0 mumol.kg-1.min-1). Therefore, under the present laboratory conditions, 54% of CHOox would be fueled by plasma glucose oxidation, whereas the remaining 46% would be derived from the combination of glycogen and lactate oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)