Objective: To determine whether aminoguaidine（AG）,a selective inhibitor of inducible nitric oxide synthase (NOS), affect the embroy development of mice, and to study the mechanisms of iNOS affecting on pregnancy. Methods: AG(10 mg*kg－1*d－1 or 20 mg*kg－1*d－1) were injected subcutaneously to mice from day 3，7，14. kill mice at day19. The weight of fetus and placenta, the number of fetus and fetal resorptions were measured. Histological changes of placenta and umbilical cord were observed. Immunohistochemistry and NADPH histochemistry were adopted to study NOS activity of placental and umbilical cord respectively. Results: The conceptus in the uterus were resorbed in the early pregnancy. In the early and middle pregnancy the weight of pregnant mouse and the number of fetus of AG groups was decreased. The number of fetal resorptions was increased. But the weight of fetus and placenta were not affected. The expression of iNOS in the placenta were no difference significantly between the two groups with image analysis. Conclusion: AG inhibits the implantation and growth of embryo in the early and middle pregnancy, buy doesn't affect the formed fetus and placenta significantly.