An in vitro experiment showed that the skin expanders were permeable to metronidazole and procaine. Twenty kidney shaped skin expanders were divided into four groups. Group 1. 100ml 0.2% metronidazole solution was injected into the expanders and the expenders were immersed in a flask filled with 100ml saline solution, and then were placed in a hermetically sealed glass chamber; Group 2.the whole procedure was the same as that of Group 1 except the expander was previously boiled in water for 30 minutes; Group 3. 100 ml 2% procaine was injected instead of metronidazole, other step was the same as that ofgroup I; and Group 4. the whole procedure was the same as that of Group 2 except the solutioninjected was 2% procaine. The concentration of metronidazole and procaine in the surrounding saline was measured at 1st, 2nd, 4th, 16th, 24th, 48th, 72nd 120th hours. The rate of diffusion of a drug was highest at 2 and 4 hous. The rate of diffusion was inversely proportional to its molecular weight, i.e., the smaller the molecular weight the greater the permeability. In view of this, during the process of expansion, metronidazole and procaine would diffuse out of the expander which might be beneficial for preventing infection and controlling pain.