The energy balance and nutrient selection strategies of 30-day-old Wistar rats offered a reference pellet and a seven-item cafeteria diet were studied in two consecutive 15-day periods: 30-45 and 45-60 days after birth. Cafeteria-fed rats grew faster, incorporating more fat and water, but a similar amount of protein to reference-fed animals. In the second 15 days all rats ate less and produced less heat than in the first 15 days. Reference-fed rats also deposited less energy in their bodies, in contrast to the tendency towards higher carcass energy deposition in cafeteria-fed rats. Cafeteria-fed rats selected much more fat and sugars than controls, with similar protein and less starch; in the second period studied, cafeteria-fed rats significantly increased their sugar consumption, with no change in fat or protein. It is suggested that the switch to selecting more sugars may be an essential factor in the shift towards increased fat deposition at the expense of heat production in cafeteria-fed rats.