In 1974 Pittet and coworkers measured the thermic effect of glucose and amino acids by both direct and indirect calorimetry. Their results have been re-analysed in this paper. Only about 20 per cent of the substrate ingested was combusted during the period of observation (2 1/2 h). The change in enthalpy due to combustion, as measured by indirect calorimetry, can be divided into two parts. Direct calorimetry shows that about 40 per cent represents heat lost from or stored in the body; the form in which the remainder is stored cannot be evaluated from the data, but it is likely that it is in chemical form, resulting from reactions that increase the enthalpy content of the substrate that was not combusted within 2 1/2 h.