[en] Objective: To observe the individual and combined effects of exposure to γ rays and hyperthermia on the development of embryonic brains. Methods: the pregnant LACA mice were exposed to 1.0 Gy ⁶⁰Co-γ rays, 42 degree C hyperthermia for 10 minutes or the two treatments combined together on day 9 of pregnancy. The females were sacrificed on day 18 of pregnancy and the fetuses were gained by cesarean section. The appearance of fetuses was observed and, then, the weight of fetal brains, the cell number of whole brains, the contents of nucleic acid and protein in brain tissue and the activity of acetylcholine esterase (AChE) in brain tissue as a marker for cholinergic neurons were determined. Results: Nervous tube defects did not occur in all groups. Compared with the control group, all the indices determined significantly declined in the radiation group while the cell number of whole brains and the AChE activity in brain tissue significantly decreased in the hyperthermia group. In the group of hyperthermia in advance, 4 hours later, followed by exposure to radiation, the AChE activity in brain tissue was significantly higher than the single radiation group. In the group of prior radiation exposure, 4 hours later, followed by hyperthermia, all the indices did not present significant difference from the single radiation group. Conclusion: The effects of 42 degree C hyperthermia for 10 minutes on the development of mouse embryo's brains are much weaker than 1.0 Gy γ radiation. It seems that the hyperthermia in advance can induce mouse fetuses to produce the cross adaptability to the following exposure to radiation. Exposure to γ radiation followed by hyperthermia does not present and additive action or a synergistic action