A study was conducted using four preruminant calves to determine the contribution of portal-drained viscera, liver, and hindquarters to circadian changes in total energy expenditure, after removing variations due to behavioral patterns. Indirect calorimetry and in vivo arterio-venous techniques were used. Standing time was longer (P < .01) after the meals and shorter (P < .01) at night. These variations were associated with higher (P < .01) energy cost of standing immediately after the meals and lower (P < .01) ones at night. When these behavioral effects were removed, total energy expenditure of lying calves was shown to be stable between the morning and evening meal, to increase by 11.5% and remained elevated during the 6 h after the evening meal, and to reach the lowest values at night. Portal-drained viscera and liver contributed 32.8 to 53.7% and 29.1 to 32.2%, respectively, to the circadian variations calculated for calves that were always standing. Changes in splanchnic tissue energy expenditure resulted from combined modifications in blood flow and O-2 extraction rate. The contribution of hindquarters could not be clearly established. Overall, portal-drained viscera, liver, and hindquarters contributed 17.2, 12.8, and 18.0%, respectively, to total energy expenditure of standing calves. Their respective in vivo metabolic activities averaged 1.08, 2.10, and .25 mu mol of O-2 consumed.min(-1).g(-1) of fresh tissue.