It has been reported that hepatic microsomal cytochrome P450(CYP) 2E1 is responsible for the metabolism of chlorzoxazone(CZX) to 6-hydroxychlorzoxazone. The present study was undertaken to assess the possible interaction of rutaecarpine, an alkaloid originally isolated from the unripe fruit of Evodia rutaecarpa, with CZX. Male Spraque-Dawley rats were administered with 80 mg/kg/day of oral rutaecarpine for three consecutive days. Twenty four hr after the pre-treatment with rutaecarpine, the rats were treated with 20 mg/kg of intravenous CZX. Rat hepatic microsomes isolated from rutaecarpine-treated rats showed greater(50% increase) activity of p-nitrophenol hydroxylase(a marker of CYP2E1) when compared with the control rats. Compared with control rats, the AUC of CZX was significantly smaller(84% decrease) possibly due to significantly faster CL(646% increase) in rats pretreated with rutaecarpine. This could be, at least partially, due to induction of CYP2E1 by rutaecarpine.