Pregnant rats were used for experiment at 20 th day after gestation, which was determined by the presence of a vaginal plug after mating. Sodium selenite[⁷⁵Se]solution was given intravenously to a number of pregnant rats and the distribution pattern of ⁷⁵Se in the mother and fetus was determined by measuring radioactivities 1 hr after injection. In the mother rats, high concentration of ⁷⁵Se was found in the liver, kidney, pancreas and lung, the concentration being twice higher than that in the blood. The transfer of ⁷⁵Se to fetus was not so marked but radioactivity 5% of the dose was found in the whole body of a litter. High concentration of ⁷⁵Se was observed in the liver, adrenal and kidney of the fetuses, concentration of ⁷⁵Se in the bone, spleen and gastro-intestinal tract was the same as that in the whole body of fetuses.