OBJECTIVE: To probe into the effects of letrozole on the endometrial receptivity and expression of vascular endothelial growth factor（VEGF） and endothelial nitric oxide synthase（eNOS） in elderly female SD rats. METHODS: The SD rats were divided into control group

model group

letrozole low-dose（LE-L） group

letrozole medium-dose（LE-M） group

letrozole high-dose（LE-H） group

with 12 rats in each group. Except for the control group

the other groups were given gonadotropin releasing hormone agonist + urogonadotropin + human chorionic gonadotropin to construct ovarian hyperstimulation models. After successful modeling

rats in the LE-L group

LE-M group and LE-H group were gavaged with letrozole at the doses of 0.13 mg/kg

0.26 mg/kg and 0.52 mg/kg

respectively

the control group and model group were gavaged with body volume of 0.9% sodium chloride solution. On the 6 th day of pregnancy

the rats were anesthetized and the abdominal cavity was dissected to observe the embryo implantation of blastocyst in uterus

the embryo implantation rate and the average number of implanted blastocyst were recorded; hematoxylin-eosin staining was used to detect pathological changes in the endometrium of rats; scanning electron microscopy was used to observe the expression of pinopode in the endometrium of rats; quantitative reverse transcription PCR was used to detect the expression of VEGF mRNA and eNOS mRNA in the endometrium; immunohistochemistry was used to analyze the expression of VEGF and eNOS in endometrium of rats. RESULTS: Compared with the control group

the endometrium of rats in the model group developed slowly

with thinner endometrium

less blood vessels

karyopyknosis

less number of pinopode

significantly lower embryo implantation rate

average number of implanted blastocyst

VEGF mRNA level

eNOS mRNA level and mean optical density values

with statistically significant differences（P<0.05）. Compared with the model group

the endometrium of rats in the LE-L group

LE-M group and LE-H group were hyperplastic and thickened

with loose and edematous lamina propria

more blood vessels

increased expression of pinopode

significantly higher embryo implantation rate

average number of implanted blastocyst

VEGF mRNA level

eNOS mRNA level and mean optical density values

which presented a dose-dependent

with statistically significant differences（P<0.05）. CONCLUSIONS: Letrozole can improve the endometrial receptivity

increase the embryo implantation rate and average number of implanted blastocyst

which may be related to the promotion of elevated expression of VEGF and eNOS.