Abstract: The aims of this study were to determine the developmental stage when HSP 70 was synthesized in the mouse embryo induced by heat shock in vitro and to study the effect of heat shock on the ultrastructural changes of the embryo in the corresponding period. The zygotes and embryoes at the 2cell, 4cell, 816cell and blastular stages were treated with heat stress at the temperature of 37, 39 and 41 ℃, respectively, to determine the expression of HSP70 by the method of immunofluorescence and cytochemistry. The mouse embryos at the early blastular stage were treated separately in vitro at 39 and 41 ℃ for 2 h. Then the embryos were collected at 0 and 2 h after heat shock to make ultrathin section; the transmission electronic microscope was used to observe the change of embryo ultrastructure. The expression of HSP70 was different at different embryonic development stages. There was a negative (-) expression in the 37 ℃ group of all stage embryo. However, in the 39 ℃ group of embryos, there was a weak expression (+) of HSP70 in the 816cell stage and a positive expression (++) at the blastular stage. In the 41 ℃ group, HSP70 was weakly expressed (+) at the 4cell stages and positively expressed (++) both at 816cell and blastular stages. There was a significant change in the ultrastructure of early blastula induced by both 39 and 41 ℃ heat shock compared to the control group. The ultrastructural change of embryo organelle treated in 39 ℃ group recovered to normal by culture at 37 ℃ for 2 h. However, the recovery was not occurred in the embryos treated at 41 ℃. These results indicate that the HSP70 is induced to synthesize in the embryos from 4cell stage and is mostly synthesized at the blastular stage; heat shock changes subcellular structure of the embryo, which appear to have degenerative changes, however, the mild changes are reversible.