The effects of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) on drug-metabolizing enzymes were studied in male and female rats. 1,2,3,4-TCDD (25, 50, 100 and 200 mumol/kg) was administered by i.p. injection once. Among the cytochrome P-450 (P450)-mediated monooxygenase activities tested, 7-ethoxyresorufin O-deethylase (EROD) activities in both male and female rats, which are associated with CYP1A1, were remarkably induced by all doses of 1,2,3,4-TCDD. The relative induction to each control activity were from 3.0- to 24.5-fold and from 2.2- to 16.5-fold, respectively. Also, 1,2,3,4-TCDD increased other CYP1A-mediated monooxygenase activities such as 7-ethoxycoumarin O-deethylase (ECOD) and 7-methoxyresorufin O-demethylase (MROD) in male and female rats dose-dependently (1.4- to 4.3-fold). Western immunoblotting showed that the levels of CYP1A1 and CYP1A2 proteins in liver microsomes were increased by 1,2,3,4-TCDD. Although the activities of other P450-mediated monooxygenases, namely 7-pentoxyresorufin O-depentylase (PROD), 7-benzyloxyresorufin O-debenzylase (BROD), aminopyrine N-demethylase (APND) and nitrosodimethylamine N-demethylase (NDAND) in both male and female rats were induced at high doses (> or = 50 mumol/kg) of 1,2,3,4-TCDD, the relative level was low compared with those of the CYP1A-mediated monooxygenase such as EROD, ECOD or MROD. In addition to P450-mediated monooxygenase, there was significant induction in the activities of the Phase II drug-metabolizing enzymes, UDP-glucuronyltransferase (UGT) activities towards 4-nitrophenol (4-NP) and 7-hydroxycoumarin (7-HC) and glutathione S-transferase (GST) towards 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB) and DT-diaphorase. These results indicate that 1,2,3,4-TCDD induces both Phase I (CYP1A-mediated monooxygenase) and Phase II drug-metabolizing enzymes (UGT, GST, DT-diaphorase) in the male and female rat liver, and that the alterations of drug-metabolizing enzyme are characteristic of PCDD toxicity.