In order to determine the effect of selenium supplementation on protein synthesis in rat liver, the rate of incorporation of (3H)-leucine into protein by isolated hepatocytes, liver mitochondria and post-mitochondrial supernatant derived from four groups of rats fed diets supplemented with 0, 0.25, 0.35 and 0.40 mg/kg selenium as selenite were investigated. In addition, the alteration in nucleic acid, lipid peroxides and glutathione peroxidase in hepatocytes from the same liver were also examined. By the end of feeding, the rates of amino acid incorporation, ribonucleic acid contents and glutathione peroxidase activities were significantly higher in hepatocytes from the 0.25, 0.35 and 0.40 mg/kg Se diet groups compared with the unsupplemented group. With increasing selenium supplementation, the increments of amino acid incorporation activity, RNA content as well as glutathione peroxidase all together plateau at approximately 0.25 mg/kg Se level of selenium supplementation. The rates of amino acid incorporation into protein in liver mitochondria and post-mitochondrial supernatant and RNA/DNA ratio in liver homogenates derived from the 0.25 mg/kg Se group were increased as compared to that from the unsupplemented group; concomitantly the increment of glutathione peroxidase activities and the reduction of malondialdehyde in liver were also found in the 0.25 mg/kg Se group. The results suggested that selenium supplementation at a 0.25 mg/kg level was sufficient to stimulate amino acid incorporation into protein in hepatocytes, mitochondria and post-mitochondrial supernatant from rat liver, and the increases in incorporation were also consistent with increments of glutathione peroxidase activities and decrease of malondialdehyde.