Nine subjects (four women and five men) consumed the energy required to maintain body weight for 3 weeks (control) and 1000 kcal per day less than the energy required to maintain body weight for 6 weeks (undereating). Ordinary foods were used to provide a mixed diet which was standardized as to composition and analysed by bomb calorimetry and by proximate analysis. During the third week of the control period and the third and sixth weeks of the undereating period, food, faecal and urine samples were collected for calculations of the metabolizable energy of the diets. The mean coefficients of availability of the three energy nutrients were lower than predicted by the Atwater method, between 83.8 and 88.3 for protein, between 91.6 and 93.2 for fat and between 92.5 and 94.7 for carbohydrates. The calorie to nitrogen ratio of the urine of these subjects during the balance periods varied from 4.96 to 19.05. The metabolizable energy of these mixed diets was 12 to 24 per cent less than the energy calculated from US food tables. Formulas proposed to calculate the metabolizable energy of diets differed from our analysed metabolizable energies by 6.4 per cent (Miller & Payne, 1959), 6.8 per cent (Southgate, 1975) and 0.7 per cent (Miller & Judd, 1984).