The purpose of this study was to determine the effect of dietary selenium on selenoprotein P concentration. Selenoprotein P was quantitated in plasma by radioimmunoassay. Selenium-dependent glutathione peroxidase activity in plasma and liver 105,000 x g supernatant was measured for comparison. Weanling male rats were fed a selenium-deficient diet or a control diet that contained 0.5 mg selenium/kg as Na2SeO4. The concentration of selenoprotein P fell at approximately the same rate in the rats fed the selenium-deficient diet as did plasma glutathione peroxidase activity. Groups of weanling rats were fed different levels of selenium for 8 wk. Selenoprotein P concentration was proportional to dietary selenium level up to 0.1 mg/kg and was a greater percentage of control values than was glutathione peroxidase activity. No increment in selenoprotein P concentration occurred between 0.1 and 0.5 mg selenium/kg diet. These results indicate that the concentration of selenoprotein P in the plasma is directly dependent on selenium supply in the diet up to 0.1 mg/kg. There is overlap between the dietary selenium ranges in which selenoprotein P concentration and glutathione peroxidase activity increase, but the selenoprotein P range is lower than the glutathione peroxidase range.