The effects of progressive reduction in food intake from 1.6 X maintenance (1.6 M) to approximately maintenance (M') and then to zero (fasting) on energy expenditure and leucine kinetics were examined in Hereford x Friesian finishing beef steers. Estimates of whole body protein synthesis and protein oxidation were obtained from the specific radioactivity of free leucine in blood and of exhaled carbon dioxide during continuous infusions of [1-14C]leucine. Protein synthesis contributed a minimum of 0.19 to total heat production across all three intakes. The apparent efficiencies with which synthesized protein was retained were 0.28 between 1.6 M and M' and 1.04 between M' and fasting. The greater efficiency below M' reflected probable use of amino acids as energy sources during fasting, which would be spared as soon as feed was available. Nitrogen derived from protein oxidation made a minor contribution to urine N at both 1.6 M (0.45) and M' (0.36) but provided a significant proportion to the increment in urine N between intakes (0.68). Amino acid absorption, estimated indirectly as the sum of protein oxidation and protein retention, represented only 0.28 of N intake at M' and 0.38 at 1.6 M but the contribution to the increment in N intake between these two diet levels was greater (0.56). The estimated efficiency of utilization of absorbed amino acids between M' and 1.6 M was 0.45.