By the use of invasive techniques, skeletal muscle has been shown to contribute to thermogenesis induced by glucose in humans. In an attempt to study this phenomenon by a non-invasive method, this study investigated intracellular high-energy phosphorous compounds in calf muscle by 31P MR spectroscopy during an oral glucose load in healthy lean subjects. The inorganic phosphate concentration increased gradually (P less than 0.05) after glucose intake. The phosphocreatine/inorganic phosphate rate decreased (P less than 0.05) and the estimated ADP concentration increased. ATP and intracellular pH remained unchanged after the glucose administration. No changes were seen in the control experiments. The processes responsible for the decreased energy state of the skeletal muscle cell may be an obligatory conversion of glucose to glycogen. Also, facultative processes, such as sodium/potassium pumping and substrate cycles stimulated by the sympatho-adrenal system, may be partly responsible.