Aim To investigate the effects of chronic unpredictable mild stress(CUMS) on platelet(PLT)5-hydroxy tryptamine(5-HT) in rat fed with high fat diet(HFD) Methods 48 male SD rats were randomly and averagely divided into four groups: control group,CUMS group,HFD group and HFD+CUMS group,with 12 rats in each group.The rats in control group and CUMS group were fed with basic food and were once intraperitoneally injected with sodium chloride,and then fed with sodium chloride by intragastric administration once every day.The rats in HFD and HFD+CUMS group were fed with high fat diet and were once intraperitoneally injected with VitD3 600000 U/kg,and then fed with propylthiouracil by intragastric administration once every day.The rats in CUMS and HFD+CUMS group were given CUMS from the 3rd week to 9th week.At the end of 9th week,platelet 5-HT in the serum were analyzed. Results The platelet 5-HT of the rats was 0.272±0.332 μmol/109 PLT,0.137±0.184 μmol /109 PLT,0.502±0.409 μmol /109 PLT,0.187±0.216 μmol/109 PLT in control group,CUMS group,HFD group,HFD+CUMS group respectively.Factor analysis showed that HFD had no significant effect on platelet 5-HT(P=0.177),while CUMS significantly decreased the concentration of platelet 5-HT(P=0.035),there was no interaction between HFD and CUMS administration on platelet 5-HT level(P=0.382). Conclusion CUMS decreased platelet 5-HT level in serum.But there was no interaction between HFD and CUMS on platelet 5-HT level in serum.