Relation between dose and absorption, and excretion of selenium was examined by short term repeated oral administration of labeled sodium selenite to rats. Male rats were divided into 4 groups of 8 rats each. To 3 groups of them, Na275SeO3 solutions (0.1 μCi/0.3 mg/2 ml/kg, 0.1 μCi/1.0 mg/2 ml/kg, 0.1 μCi/3.0 mg/2 ml/kg) were given orally every day for 35 days, and remaining 1 group was used as control. During the repeated administration, gain in body weight, whole body accumulation and excretion to urine and feces of selenium were examined. Gain in boody weight was suppressed in 3.0 mg/kg group compared with control group. At 35th day, weight of liver and kidney were markedly increased in 1.0 and 3.0 mg/kg groups, and histological changes were found only in 3.0 mg/kg group. During the time of repeated administration, the amount of whole body accumulation of 75Se was not proportional to the dose, and the rate of daily excretion of 75Se in the experimental groups was roughly proportional to the dose, and declined with the lapse of days in all the groups. After the termination of repeated administration of sodium selenite for 35 days, the radioactivity was excreted mainly via urine. Elimination of 75Se was rather rapid for 10 days, became slower, and the whole body count decreased along an exponential line. The biological half lives of absorbed labeled sodium selenite were calculated from the lines obtained by plotting the whole body counting on semi-logarithmic scale, and the values in 0.3 and 3.0 mg/kg dose groups were 26 and 24 days, respectively. At 35th day, higher accumulation of 75Se was found in the kidney, epididymis, spleen, liver, and blood in this order, but a remarkable decrease in liver and kidney was obseved 65 days after termination of the administration. In blood and hair, concentration of selenium was directly proportional to the dose.