To study the effects of heat and ionizing radiation on mouse embryos, the LACA mice were exposed to 42℃ for 10min and 1.0Gy ,60,Co γ rays on the 9th day after gestation, and development of mouse embryos were observed and the contents of DNA, RNA and protein in the brains of mouse embryos were measured on the 18th day after gestation. Results demonstrated that 1.0Gy ,60,Co γ rays irradiation, compared with control, could induce the development retardation for mouse embryos, and the contents of nucleic acids and protein in brains of mouse embryos irradiated were lwss. Meanwhile 42℃ exposure induce insignificant or less effects. The mouse group with 42℃ treatment followed by 1.0Gy ,60,Co irradiation showed the efffects at the level between the groups with 42℃ and 1.0Gy irradiation. Thus, exposure of mouse embryos to 42℃ for 10min prior to irradiation may be could induce their adaptive response to irradiation.