The authors has observed the effect of X-ray irradiation on the dentin matrix formation of the albino rat fetuses. The lower abdomen of the pregnant ratswere exposed to X-ray on the 9 1/2th day of gestation, respectively 150, 250 and 350 rads. The fetuses of the right sides of the same pregnant rats which were not exposed to X-ray were as controls. The results were as follows: 1) In the 150 rads irradiated fetuses, predentin formation was identical with control groups, but the arrangement of odontoblasts was distorted, subodontoblastic layer was condesed with pulp cells and blood capillaries were enlarged. 2) In the 250 rads irradiation, dentin matrix was imperfact or osteodentin was occured. Short columnar or cuboidal odontoblasts were presented and pulp cells were dispersed. Blood capillaries were cogested. 3) 350 rads irradiated fetuses showed osteodentin matrix and numerous degenerated odontoblasts. Their dental papilla showed reticular atrophy and enlarged capillary.