The thermal environments in known markedly influence the energy and water needs and balances in livestock. In this study, body composition of sheep was evaluated when they were fed with the same feed, both quantitatively and qualitatively, but kept in either 0,20 or 40oC. Fifteen mature ewes, 45 to 50 kg at the start of the experiment, were divided into three groups randomly and kept in individual metabolis cages in controlled ~emperature rooms ( 0,20, or 40°C) and were lit continuosly with fluorescent lights. A diet of 1000 g of pelleted feed was offered once daily at 1430 hour. The feed contained 79% roo led barley, 18% soybean meal, 1% limestone, 2% perma pell (pellet binder), and 0.002% of vitamin A, D and E mix (approximately equal to the maintenence requirement for the sheep in the cold treatment). Drinking water and a mineralized salt block were available ~«! jj,}!itum. The sheep were shorn at approximately six weekly interva Is and were weighed biweekly. After five months of acclimation, each sheep was injected with triated water (TOH) in order to estimate their. body composition. The diet had a moisture content of 13.7%, 17.5% crude protein and 15.3 I1J/kg of dry matter. Two sheep kept in 400C thermal environment were excluded from the trial due to an accident of a broken leg and a sickness which was not related to the trial. At the end of the trial, average liveweights were 50,66, and 70kg for animal group kept in 0,20 and 40oC, respectively. The respective groups had 14.1, 12.8, and 13.9% body protein contents, and they had 15.7, 27.5, and 16,8% body fat contents, prodtein contents, and they had 15.7, 27.5, and 16.8 , body fat content, and they had 69.9, 59, and 68.8% body water contents. The warmer the thermal environment, the lower the biological halflife (Tl/2) (207, 157, and, 152 hours), the lower the retention of body water (12.4, 9.4, and 9.1 hours), and practically the higher body passage per-unit time (2.9, 4.6, and 6.0 litre/day). The data suggest that energy body reserves of animal kept in 20C thermal environments are most likely higher than those kept in others thermal environments. M.Winugroho*,Y. Saepudin**V.A. Walker*, B.A. Young*, dan* Departemen of Animal Science, Faculty of Agriculture and Forestry, University of Alberta, Canada. ** Balai penelitian Ternak