Objective To investigate effects of mifepristone (MP) on the development and ultra- structure of mouse blastocysts in vivo Methods Sixty female mice were equally divided into 4 groups: control group (group A), 1.9 mg/kg MP group (group B), 5.6 mg/kg MP group (group C), and 16.8 mg/kg MP group (group D). The female mice of 4 groups undertook a superovulation method. The development of obtained blastocysts was evaluated, and ultrastructural changes of the blastocysts were observed by transmission electron microscope. Results In comparison with group A, the development rate of blastocysts in group B showed no difference (P>0.05), while the development rate of blastocysts in both group C and group D was significantly lower (P<0.05). With doses of 5.6 mg/kg and 16.8 mg/kg, the blastocysts showed granular appearance of the cytoplasm, irregular cell borders, enlarged perivitelline space and degeneration. Ultrastructure of the blasto- cysts in group B was similar to group A, except a little number of fat droplets in the cytoplasm. In group C, the microvilli on apical surface was decreased in number or even disappeared, mitochondria were under developed, a lot of filamentous crystals were found in the cytoplasm. Cellular junctions were defected. In group D, the blasto- cyst cells were irregular in shape, mitochondria were frequently vacuolated, the nucleolus was enlarged, nuclear membrane was ruptured, and chromatin was slack. Conclusion MP could damage to the ultrastructure of mouse blastocyst, and was responsible for the inhibition of blastocyst development. The inhibitory effect of MP would be in a dose-dependent fashion. Objective To investigate effects of mifepristone (MP) on the development and ultra-structure of mouse blastocysts in vivo Methods Sixty female mice were divided into 4 groups: control group (group A), 1.9 mg / kg MP group 5.6 mg / kg MP group (group C), and 16.8 mg / kg MP group (group D). The female mice of 4 groups undertook a superovulation method. The development of obtained blastocysts were evaluated, and ultrastructural changes of the blastocysts were observed Results of the development rate of blastocysts in group B showed no difference (P> 0.05), while the development rate of blastocysts in both group C and group D was significantly lower (P <0.05) . With doses of 5.6 mg / kg and 16.8 mg / kg, the blastocysts showed granular appearance of the cytoplasm, irregular cell borders, enlarged perivitelline space and degeneration. Ultrastructure of the blasto- cysts in group B was similar to group A, except a little nu mber of fat droplets in the cytoplasm. In group C, the microvilli on apical surface was decreased in number or even disappeared, mitochondria were under developed, a and sls were in the cytoplasm. Cellular junctions were defected. In group D, the blasto- cyst cells were irregular in shape, mitochondria were frequently vacuolated, the nucleolus was enlarged, nuclear membrane was ruptured, and chromatin was slack. Conclusion MP could damage to the ultrastructure of mouse blastocyst, and was responsible for the inhibition of blastocyst development The inhibitory effect of MP would be in a dose-dependent fashion.