Eleven Holstein bull calves 35 d of age were assigned to one of three treatment groups: (1) W72, warm environment (20°), 72 g feed/kg body weight (BW)0.75 per d, (2) C72, cold environment (−5°), 72 g feed/kg BW0·75 per d, or (3) C90, cold environment (−5°), 90 g feed/kg BW0·75 per d. Fractional synthesis rates (FSR) of protein in the rumen wall, rumen papillae, omasum, duodenum, kidney, liver,heart, longissimus dorsi, biceps femoris and skin were determined following a continuous infusion of[3H]phenylalanine. Phenylalanine flux was elevated in both groups of cold-adapted calves. FSR of protein in the two muscles and skin were reduced along with N retention in the calves in the C72 group compared with the other two groups. Muscle protein degradation, estimated from urinary Nτ-methylhistidine excretion, tended to be elevated in both groups of cold-adapted calves. Reduced protein synthesis and increased protein degradation in the C72 group contributed to reduced muscle protein gain. It appears that when feed intake is limited in cold-adapted animals, muscle and skin have a lower priority for nutrients than other organs and tissues, resulting in reduced protein synthesis, it seems unlikely that thermogenesis due to enhanced protein synthesis contributed to the increased heat production in the cold.