In order to investigate the effect of dietary selenium on the activities of glutathione peroxidase and deiodinase and the least selenium requirement for their optimal activities, fifty four male weanling Wistar rats of 50-60g body weight were randomly divided into 9 groups and were fed with semisynthetic diets of different selenium level for 20 weeks. The selenium in the diets were 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, 0.2 and 5 mg/kg diet respectively. The body weight of ratsin the ninth group was different significantly from the seventh and eighth group, but those in the other eight groups were not significantly different. The activities of liver glutathione peroxidase in 0.1, 0.2 and 5 mg/kg groups were the highest among the nine groups. So at least 0.1 mg/kg diet is required for its normal activity. The activities of the liver type I deiodinase of nine groups were high from 0.05 to 0.2 mg/kg diet groups and at least Se 0.05 mg/kg diet is required for its normal activity.