Objective To study the effect and mechanism of Pingwei Powder （Powder for gastrointestinal diseases） on TCM syndrome of damp retention in the middle jiao（-energizer）. Methods Fifty SD rats were randomized into blank group

blank group with medication

model group

Pingwei Powder group

and natural recovery group

with 10 in each. The models of damp retention in the middle jiao syndrome were prepared with the imitated pathogenic factors including excessive exogenous damp

improper food-intake

and emotional stress. Pingwei Powder Decoction was given to the Pingwei Powder group and blank medication group 10mg/kg by gastric perfusion; normal saline 10mg/kg was given to the blank group and natural recovery group. Samples were collected 3 day later. The immunohistochemical technique was used to measure the distribution of aquaporin 2 （AQP2） in the gastrointestinal tissues

and the enzyme-linked immunosorbent assay was adopted to measure its content. Results In the model rats of damp retention in the middle jiao syndrome

the AQP2 expression in the intermucosa of body of stomach and the submucous tissue in the middle part of large intestine was decreased

while in the tissues of middle part of small intestine

the mucosa of large intestine

and the submucous tissue in the ending part of large intestine it was increased. Pingwei Powder made the AQP2 expression in the intermucosa of body of stomach

the mucosa of the middle part of large intestine

and the submucous tissue in the ending part of large intestine enhanced and its expression in the submucous tissue of the middle part of large intestine inhibited. Conclusion The expressional distribution spectrum of AQP2 with its pathological characteristics in the gastrointestinal tissue may be one of the molecular bases in the damp retention in the middle jiao syndrome. The intervention of Pingwei Powder on its expression may be one of the molecular mechanisms of this formula in treating this syndrome.