Medaka (Oryzias latipes) embryos at 25-26 and 28-30 stages were irradiated with a single acute dose of 10 Gy of X-ray, which is lower than the LD50 of the embryos. The effects on developing brains were examined under a stereomicroscope in living embryos until hatching. All the irradiated embryos survived; however, from 6 to 35 h after X-ray irradiation, massive clusters of optically opaque and round cells were observed either in the entire brain region (when irradiated at 25-26 stages) or mainly in the optic tectum (when irradiated at 28-30 stages). Histological examination and TUNEL showed that these cells are clusters of dead cells. These dead cell clusters disappeared thereafter, and the irradiated embryos continued to develop apparently normally. The grown irradiated embryos, however, had smaller brains and eyes than the nonirradiated control embryos. At hatching, the irradiated embryos exhibited histological abnormalities in the brain, particularly in the torus longitudinalis, and in the retina, although most of them hatched normally and survived. The results indicate that brain cell death and a reduced brain size can be observed in living irradiated embryos, and suggest that the medaka embryo is useful for screening the developmental neurotoxicity effects of various hazardous factors.