Aim:To investigate the effect of mifepristone(RU486)as a single dose onpinopodes expression in the endometrial surface of mice at the time of implantation.Methods:Pregnant mice in the treated four groups received mifepristone subcu-taneously(0.1 mg)between 07:00 and 08:00 AM on Pd(day of pregnancy)1,Pd2,Pd3,and Pd4.Pregnant mice in the non-treated group were used as controls.Theuterine horns were collected randomly from two mice in each group between 21:30and 22:00 PM on Pd4,and from another two mice of the same group between 09:30and 10:00 AM on Pd5.The specimens were examined by scanning electron mi-croscopy for the detection of pinopodes.Results:When mifepristone was givenon Pdl,developing and fully developed pinopodes were observed,but the ex-pression was markedly reduced compared to the control group.When mifepristonewas administered on Pd2,only a few developing pinopodes were present.Whenmifepristone was administered on Pd3,developing pinopodes were observed.When mifepristone was administered on Pd4,different development stagepinopodes were present in specimens collected between 21:30 and 22:00 PM onPd4,but no pinopodes was observed in specimens taken between 09:30 and 10:00AM on Pd5.Conclusion:These findings suggest that administration of a singledose of RU486 subcutaneously on Pd1,Pd2,Pd3,and Pd4 might play a role ininhibiting development and maturation of endometrium,hence affecting embryoimplantation in mice. Aim: To investigate the effect of mifepristone (RU486) as a single dose on pinopodes expression in the endometrial surface of mice at the time of implantation. Methods: Pregnant mice in the treated four groups received mifepristone subcu-taneously (0.1 mg) between 07: 00 and 08:00 AM on Pd (day of pregnancy) 1, Pd2, Pd3, and Pd4.Pregnant mice in the non-treated group were used as controls.Theuter horns were collected randomly from two mice in each group between 21: 30and 22:00 PM on Pd4, and from another two mice of the same group between 09:30 and 10:00 AM on Pd5. The specimens were examined by scanning electron microscopy for the detection of pinopodes. Results: When mifepristone was given on Pdl , developed and fully developed pinopodes were observed, but the ex-pression was markedly reduced compared to the control group. Who mifepristonewas administered on Pd2, only a few developing pinopodes were present .Whenmifepristone was administered on Pd3, developing pinopodes were observed. Who mifepristone was administered on Pd4, different development stage pinopodes were present in specimens between 21:30 and 22:00 PM onPd4, but no pinopodes was observed in specimens between 09:30 and 10:00 AM on Pd5.Conclusion: These findings suggest that administration of a singledose of RU486 subcutaneously on Pd1, Pd2, Pd3, and Pd4 might play a role in inhibiting development and maturation of endometrium, thus affecting embryoi implantation in mice.