Abstract: Cyclophosphamide (CPA) is the first-line chemotherapy for many tumors, but its overdose will lead to hepatotoxicity. This study aims to investigate whether the combined administration of oxymatrine (OMT) with CPA will aggravate the hepatotoxicity induced by CPA and its engaged mechanism. The expression of hepatic Cyp2b10 mRNA and CYP2B10 protein was detected by qPCR and Western blot in mice at different times after OMT (100 mg·kg^-1) administration. Mice were given with different doses of OMT (intragastric administration, ig) every day. At the same time, CPA (200 mg·kg^-1) was also intraperitoneally injected into mice every other day. After 10 days, serum alanine/aspartate aminotransferase (ALT/AST) activity, the mortality of mice and hepatic mRNA expression of Cyp2b10 were detected. Furthermore, the correlation among ALT/AST activity, the mortality and Cyp2b10 mRNA expression was analyzed. All animals were received humane care according to the institutional animal care guidelines approved by the Experimental Animal Ethical Committee of Shanghai University of Traditional Chinese Medicine. The results showed that OMT itself enhanced hepatic mRNA and protein expression of Cyp2b10 (P<0.05), and increased liver enzymatic activity of CYP2B10 in mice (P<0.05). In mice treated with CPA plus OMT, OMT obviously enhanced the mortality of mice induced by CPA (from 33.3% to 58.3%). The results of serum biochemical analysis and hepatic mRNA expression of Cyp2b10 showed that OMT further enhanced the increased serum ALT/AST activity and hepatic Cyp2b10 mRNA expression in mice (P<0.05). There was a good correlation between serum ALT/AST activity and mortality or hepatic Cyp2b10 mRNA expression. These results showed that OMT could enhance hepatic Cyp2b10 mRNA expression and increase liver CYP2B10 enzymatic activity, and then promoted the metabolism of CPA, and thus aggravated CPA-induced hepatotoxicity in mice.