1. This study compared the effect of dietary supplementation with organic or inorganic selenium (Se) sources plus control amounts or large amounts of vitamin E (alpha- tocopherol acetate) in broilers raised at control ( 20 to 24 degrees C) or low ( 14 center dot 5 to 16 center dot 8 degrees C) temperatures after 2 weeks of age. 2. The following dietary treatments were used from one day old. Diet 1, the control diet, comprised a commercial diet containing 0 center dot 15 mg/ kg inorganic Se and 50 mg vitamin E/kg feed. Diet 2 was the same as diet 1, supplemented with 0 center dot 15 mg/ kg inorganic Se. Diet 3 was the same as diet 2 but was supplemented with 200 mg/ kg vitamin E. Diet 4 was the same as diet 1, but inorganic Se was replaced with 0 center dot 30 mg/ kg organic Se. Diet 5 was the same as diet 4, supplemented with 200 mg/ kg vitamin E. 3. Low temperature reduced the growth rate of broilers; however, at 6 weeks, there were no differences in the body weights of birds fed on organic Se supplemented diets housed at low or control temperature. The feed conversion ratio was significantly affected by low temperature but not by diet. The heterophil/ lymphocyte ratio was higher in chicks after one week in the cold, indicating mild stress. Blood triiodothyronine levels were significantly higher in birds after 1 and 4 weeks in the cold but thyroxin was not affected. 4. Organic Se supplementation increased relative lung weight at the control temperature, which might lead to greater respiratory capacity. Relative spleen weight significantly decreased in broilers fed diets supplemented with inorganic Se under cold conditions, a possible indication of chronic oxidative stress. 5. At the low temperature, supplementation with organic Se alone, or with inorganic Se and vitamin E increased glutathione peroxidase ( GSHPx) activity and glutathione ( GSH) concentration in the liver of broilers, which may indicate increased activity of birds' antioxidant defence against suboptimal environments.