AIM: To evaluate the effects of frying oil and Houttuynia cordata Thunb (H. cordata), a vegetable traditionally consumed in Taiwan, on the xenobiotic-metabolizing enzyme system of rodents.METHODS: Forty-eight Sprague-Dawley rats were fed with a diet containing 0%, 2% or 5% H. cordata powder and 15% fresh soybean oil or 24-h oxidized frying oil (OFO)for 28 d respectively. The level of microsomal protein, total cytochrome 450 content (CYP450) and enzyme activities including NADPH reductase, ethoxyresorufin O-deethylase (EROD), pentoxyresorufin O-dealkylase (PROD), aniline hydroxylase (ANH), aminopyrine demethylase (AMD), and quinone reductase (QR) were determined. QR represented phase Ⅱ enzymes, the rest of the enzymes tested represented phase Ⅰ enzymes.RESULTS: The oxidized frying oil feeding produced a significant increase in phase Ⅰ and Ⅱ enzyme systems,including the content of CYP450 and microsomal protein,and the activities of NADPH reductase, EROD, PROD, ANH,AMD and QR in rats (P＜0.05). In addition, the activities of EROD, ANH and AMD decreased and QR increased after feeding with H. cordata in OFO-fed group (P＜0.05). The feeding with 2% H. cordata diet showed the most significant effect.CONCLUSION: The OFO diet induces phases Ⅰ and Ⅱ enzyme activity, and the 2% H. cordata diet resulted in a better regulation of the xenobiotic-metabolizing enzyme system.