Direct Black 38, a kind of benzidine-based azo dye, is widely used as a dye for fabric, leather, cotton, cellulosic material, paper, wool, silk, and so on. Benzidine-based azo dyes are proven as a mutagen and linked to bladder cancer. In 1978, Natonal Institute for Occupational Safety and Health recommended that three widely used benzidine-based dyes (Direct Black 38, Direct Blue 6, and Direct Brown 95) should be treated as carcinogens. In this experiment, metabolism of the benzidine-based dye. Direct Black 38 was examined by using an isolated liver perfusion system. To measure the metbolites of Direct Black 38,/ 8.0 micrometer, 30.5 micrometer and 63,3 micrometer of Direct Black 38 was added into the recirculating perfusate of the isolated perfused rat liver. Samples were collected at 0, 10, 20, 30, 60, 90. 120 minute. They were treated with sep-pak and methanol, and the metabohtes were detected and quantified with high performance liquid chromatography (HPLC). Residual non-reactive dye in the perfusate and liver was reduced to benzidine and then analyzed by HPLC. Detected metabolites of ?Direct^-Black 38 were benzidine, N-acetylbenzidine, and N,N'-diacetylbenzidine. The average conentration of benzidine was 0.1 micrometer and this concentration was maintained throughout the experimental period. The average concentration of N-acetylbenzidine was 0.22 micrometer and took the same pattern of benzidine. When 30.5 micrometer of Direct BIact 38 was added to the perfusate, only. N,N'-diacetylbenzidin.e increased slightly with time. From the above results we suggest that only small amount of Dirst Black 38 might be metabolized to benzidine regardless of the s, amount of the Direct Black 38. There are some possible explanations. The liver was damaged during the perparation. And the function of the perfused liver decreased rapidly because adsorbing of dye. The benzidine, acetylbenzidine and diacetylbenzidine metabolized to other metabolites were not detected in this experiment.