Abstract: OBJECTIVE To investigate whether Ganmai Dazao Decoction (GMZ) combined with fluoxetine (Flx) treatment can alleviate depressive symptoms in chronic stress mice by modulating intestinal microbiota. METHODS Chronic unpredictable mild stress (CUMS) with solitary raise was employed to establish a chronic stress mice model. Mice body weight, sucrose preference, tail suspension test (TST), open field test (OFT), and brain 5HT level were evaluated to clarify the efficacy of GMZ (7 g/kg), Flx (10 mg/kg), or GMZ (7 g/kg)+Flx (10 mg/kg) with intragastric administration in CUMS mice. 16S rDNA sequencing was used to analyze the changes of intestinal microbial abundance and structure in CUMS mice. The effects of transplantation with GMZ+Flx group fecal bacteria on depressive behavior and the brain 5HT level in CUMS mice were assessed by fecal bacteria transplantation experiments.RESULTS The chromic stress mice model was successfully reproduced by CUMS stress with solitary raise. GMZ combined with Flx treatment could significantly increase the bodyweight (P<0.01) and sucrose preference percentage (P<0.01), reduce the immobility time of TST (P<0.01), increase the horizontal locomotor of OFT (P<0.05), and increase the level of brain 5-HT (P<0.01) in CUMS mice compared with control mice. In addition, compared with control mice, the relative abundance of Bacteroidetes in intestinal microbiota of mice in CUMS group was significantly decreased (P<0.01), and that of Firmicutes was significantly increased (P<0.01). However, the relative abundance of Firmicute was reduced notably (P<0.05) and Bacteroidetes was increased mildly in GMZ-treated mice. Furthermore, compared with CUMS mice, GMZ+Flx treatment fecal bacteria transplantation significantly increased the percentage of sucrose preference (P<0.01), reduced the immobility time of TST (P<0.05), increased the horizontal locomotor of OFT (P<0.01), and increased the level of brain 5-HT (P<0.01). Moreover, both intragastric administration and fecal bacteria transplantation experiments showed that GMZ reduced the immobility time of TST in CUMS mice remarkably, whereas Flx increased the horizontal locomotor of OFT obviously. CONCLUSION GMZ combined with fluoxetine treatment can alleviate the depressive symptoms of chronic stress mice by modulating intestinal microbiota.