In order to determine whether or not hepatic glycogen breakdown contributes to systemic glucose flux during glucose infusion, net carbohydrate oxidation (indirect calorimetry) and the total rate of glucose appearance (6,6(-2)H-glucose) were measured in six healthy women during infusion of U-13 C labelled glucose (22 mumol/kg/min). Glucose infusion completely suppressed endogeneous glucose production and increased net carbohydrate oxidation from 10.9 +/- 1.6 to 18.9 +/- 1.0 mumol/kg/min. To differentiate between the oxidation of endogenous (i.e. glycogen) and of exogenous carbohydrates, the 13CO2 production was measured and the oxidation of exogenous 13C labelled carbohydrate was calculated. For this purpose, the specific recovery factor in breath of 13CO2 issued from oxidation of uniformly labelled glucose was determined during infusions of equimolar amounts of 13C bicarbonate, 1-13C acetate and 2-13C acetate. The average recovery was 53.9 +/- 1.5%. The oxidation of exogenous carbohydrate was 20.9 +/- 0.7 mumol/kg/min. This value was slightly higher than net carbohydrate oxidation, indicating that no oxidation of endogenous, unlabelled carbohydrate, and, hence, no utilization of hepatic glycogen took place. These results indicate that (i) estimation of glucose oxidation from indirect calorimetry and tracer technology give concordant results when an appropriate factor of 13CO2 recovery in breath is used, and (ii) utilization of previously formed glycogen is inhibited during hyperglycaemia and hyperinsulinaemia.