Measurements were made of energy balance by direct calorimetry, and of nitrogen balance in groups of lean and congenitally obese ('fatty') Zucker rats at body-weights of 200 and 350 g given a highly digestible semisynthetic diet at 14-0 or 18-4 g/rat per 24 h. 2. Losses of food energy and N in faeces were very small. The fatty rats lost much more N in urine than did lean rats. Despite this the proportion of gross energy that was metabolized was 0-92 for both fatty and lean rats. 3. In all trials, fatty rats lost a smaller proportion of metabolizable energy (ME) as heat and deposited less as protein than thin rats but deposited much more as fat. 4. The amounts of ME required to deposit 1 kJ of protein and 1 kJ of fat respectively were shown by regression analysis to be 2-25 (+/-0-16) and 1-36 (+/-0-06) kJ respectively. These values agree extremely closely with recent, more tentative, estimates based on assumptions as to maintenance requirement which the present experiments were able to circumvent. It may be concluded with confidence that the energy costs of depositing 1 g of protein or fat are almost identical at 53 kJ ME/g.
