Adult Japanese quail (Coturnix coturnix japonica) which had been fed, since hatching, on either a conventional diet (low-fibre) or on the same diet diluted with 200 g oak sawdust/kg (high-fibre) took 8 to 10 d to adjust their food intake when their diets were interchanged. 2. Those fed on high-fibre weighed less than those fed on low-fibre diet, and body weight changed rapidly when the diets were interchanged. All the birds digested about the same weight of nutrients in a day, irrespective of diet, and this suggests that body weight differences were due mainly to differences in energy utilisation, birds fed on high-fibre diet having to expend more energy on feeding than those fed on low-fibre diet. 3. Gut size was greater in birds fed on high-fibre diet, presumably to accommodate the greater bulk of food eaten, and when the diets were interchanged the gut dimension of both groups changed at similar rates, reaching the appropriate sizes for the respective diets in 3 to 4 weeks.