The effect of thiabendazole (TB) on some rat hepatic xenobiotic metabolising enzymes has been investigated. Male Sprague-Dawley rats were fed control diet or diets containing 102-5188 ppm TB for 28 days. As a positive control for induction of hepatic xenobiotic metabolism, rats were also fed diets containing 1457 and 10,155 ppm butylated hydroxytoluene (BHT). Treatment with TB and BHT resulted in dose-dependent increases in relative liver weight. TB was found to be a mixed inducer of cytochrome P450 (CYP) forms in the CYP1A and CYP2B subfamilies. The administration of high doses of TB resulted in the induction of 7-ethoxyresorufin O-deethylase and 7-pentoxyresorufin O-depentylase activities, CYP1A1, CYP1A2, CYP2B1 and CYP2B1/2 mRNA levels and CYP1A2 and CYP2B1/2 apoprotein levels. In contrast, BHT was a CYP2B form inducer, increasing 7-pentoxyresorufin O-depentylase activity, CYP2B1 and CYP2B1/2 mRNA levels and CYP2B1/2 apoprotein levels. Both TB and BHT induced GSH S-transferase activities towards a range of substrates. In addition, TB and BHT markedly induced GSTP1 mRNA levels, but had only a small effect on GSTT1 mRNA levels. In summary, these results demonstrate that TB induces both phase I and II xenobiotic metabolising enzymes in rat liver. © 2004 Elsevier Ltd. All rights reserved.