Objective · To investigate the effects of gonadotropin-releasing hormone-antagonist (GnRH-ant) on the proportion and toxicity of mice uterine nature killer (uNK) cells during implantation window. Methods · Sixteen C57BL/6 mice were randomly divided into GnRH-ant group and control group, with 8 mice in each group. From the 3rd day of the estrous cycle, GnRH-ant (1.5 μg/100 g) was injected intraperitoneally into the mice of the GnRH-ant group for 7 days continuously, and the control group was injected with the same volume of normal saline at the same time point. On the 7th day, the mice of the two groups were injected with human menopausal gonadotropin (40 U/100 g). The next day, they were injected with human chorionic gonadotropin (100 U/100 g) and sacrificed after 48 h. The uterus tissues were taken out for primary digestion to obtain single-cell suspension. Flow cytometry was used to analyze the proportion of uNK cells and the expression levels of toxicity molecules perforin (Pf) and granzyme B (Gz-B). Results · Compared with the control group, the proportion of uNK cells in GnRH-ant group increased (P=0.000), the proliferation level increased (P=0.000), the apoptosis level decreased (P=0.004), and the expression of toxicity molecules Pf (P=0.000) and Gz-B (P=0.034) were up-regulated. Conclusion · GnRH-ant may up-regulate the proportion of uNK cells and enhance their toxicity in the implantation window period of mice.