Triturus pyrrhogaster embryos at various stages of development were exposed to neutron radiation, and the effects were studied externally and microscopically. For an analysis of the abnormality in the nervous system produced by the irradiation, the difference in neutron-sensitivity between the presumptive neural ectoderm and the chordamesoderm was also investigated in vitro.Neutron-sensitivity descreased from the cleavage stage to the tail-bud stage; in the latter no effect was observed. Rather high frequency of abnormality was observed in mid-blastula and early gastrula stages. The abnormalities consisted of microcephalia, acephalia, cyclopia, axial distortion, and edema. The nervous system in particular was very sensitive to neutron radiation.The irradiated presumptive neural ectoderm Wrapping non-irradiated chordamesodenn underwent cytoloysis in vitro and no differentiation occurred in it, while the irradiated chordamesoderm wrapped in the non-iyadiated ectoderm was never affected and retained its inducing and self-differentmtion ability.The results suggest that the irradiated and damaged cells release a cytotoxic agent.