One classical theory of food intake control suggests that, within limits, food intake is regulated to ensure that requirements for energy are just met. Two of the prime objectives of the calf rearer are to achieve rapid growth and early intake of solid food (to allow early weaning). The optimum concentration of energy, therefore, will be sufficiently low to maximise intake of solid food, yet high enough to maximise total intake of energy. A complication is introduced by the fact that the pre-ruminant calf receives two foods simultaneously; a daily allowance (usually fixed) of milk replacer supplemented by ad libitum access to solid starter feed. Theory predicts that intake of starter will be determined by concentrations of energy in both starter and milk replacer (assuming other nutrients supplied in excess of requirements). If the calf is able to select a level of intake of starter which allows requirements for energy to be met, rate of growth will be independent of concentrations of energy in both starter and milk replacer. Our objective was to test these predictions in a conventional calf rearing system.