OBJECTIVE To investigate the effect of the endoexpander pressure of continuous and constant pressure expansion on the drug permeability. METHODS The expanders were divided into two groups, the normal expansion and the continuous and constant pressure expansion (4.6 kPa). Each expander was filled with 0.2% Metronidazole, then the expanders were immersed wholly in normal saline and sealed totally. At several intervals over 72 hours, the surrounding saline was sampled and the drug concentration of the sample was measured respectively. RESULTS Both groups were permeable to the Metronidazole and the concentration outside the expander would reach the effective concentration in 48 hours. The drug concentration of the continuous and constant pressure expansion was higher than that of the normal one and there was significant difference between the two groups (P lt; 0.01). CONCLUSIO The endoexpander pressure in continuous and constant pressure expansion can enhance the drug permeability. In view of this, in the course of continuous and constant pressure expansion, 0.2% Metronidazole can be used to prevent and control the infection.