The author studied on the effects of x-irradiation to the development of digital malformation in gestation rats. The time-matings occured between 6 p.m. and 8 a.m. and females with copulation. plugs at 8 a.m. were isolated and properly marked for evidence of copulation. The lower abdomen of mothers were exposed to x-irradiation on the 11½th day of gestation, the critical period developing digital malformation, respectively 100, 150, 200, 250, 300 and 350 rads. At 18½th day of post-conception total 50 pregnant females were dissected and the incidence of digital malformations were obtained. Rat embryos on the 12, 13, 14, 15, 16th day of gestation irradiated by 250 rads were examined for morphogenesis of digital malformation. Digital radiating lines were examined in water and histologically by H-E stain. Supra vital stain samples by Nile-blue sulfate in 37℃ normal saline were prepared for the observation of cell necrosis regions and morphogenesis of digits. The results obtained were as follows; 1. By x-irradiation on 11th day of gestation, digital malformations of Ectrodactylia, Syndactylia Polydactylia and Hematodactylia were developed. Ectrodactylia showed the effective relationship to the amount of irradiation, however Syndactylia ans Polydactylia did not. 2. By x-irradiation, cell necrosis of digital germ was appeared markedly, but in 48 hours after irradiation was depressed to the periphery of digital germ and in 72hours after irradiation was disappeared. Digital radiating line showed marked stage of malformation in 48hours after irradiation and continued to show the same amount of physiological cell necrosis as the compared control group in 72hours. after irradiaion. But in the Syndactylia, physiological cell necrosis was not able to be recognized. 3. Ectrodactylia induced by x-irradiation was considered as the direct resoult of cell necrosis of digital origin, however, Polydactylia and Syndactylia were considered as the resoult of some effect in repair process of x-irradiation damages.