Objective To evaluate the effect of fentanyl on the expression of vascular endothelial growth factor A (VEGF-A) and matrix metallopeptidase-9 (MMP-9) in the subcutaneous tumor of human gastric cancer in nude mice.Methods Thirty SPF male BALB/C nude mice,aged 4-5 weeks,weighing 15-20 g,in which the model of subcutaneous tumor of human gastric cancer cell line MGC-803 was established,were randomly divided into 6 groups (n =5 each) using a random number table:control group (C group),normal saline group (NS group) and fentanyl 0.05,0.10,0.20 and 0.40 mg/kg groups (F1-4 groups).The mice in group C received no treatment.Fentanyl 0.05,0.10,0.20 and 0.40 mg/kg were intraperitoneally injected once a day for 14 consecutive days in F1 4 groups,respectively,while the equal volume of normal saline 1.5 ml/kg was given instead of fentanyl in group NS.The nude mice were sacrificed on 1 day after the end of administration,and the tumor tissues were obtained for examination of the ultrastructure of subcutaneous tumor (with a transmission electron microscope) and for detection of the expression of VEGF-A and MMP-9 (by immunohistochemistry and Western blot) and expression of VEGF-A and MMP-9 mRNA (by real-time reverse transcriptase polymerase chain reaction).Results No abnormality in the morphology of the subcutaneous tumor cells was observed in C and NS groups.The swollen nucleus,chromatin margination,nuclear fragmentation and apoptotic bodies were found in the subcutaneous tumor cells in F1-4 groups.Compared with group C,the expression of VEGF-A and MMP-9 protein and mRNA was significantly down-regulated in F1-4 groups (P＜0.05),and no significant change was found in each parameter mentioned above in group NS (P＞0.05).There was no significant difference in the expression of VEGF-A and MMP-9 protein and mRNA among F1-4 groups (P＞0.05).Conclusion The mechanism by which fentanyl inhibits the growth and metastasis of subcutaneous tumor cells of human gastric cancer is related to down-regulation of VEGF-A and MMP-9 expression in nude mice.