1. The herbicides butachlor (2-chloro-2',6',diethyl-N-[buthoxymethyl] acetanilide) and pretilachlor (2-chloro-2',6'-diethyl-N-[2-propoxyethyl] acetanilide) are widely used in Asia, South America, Europe and Africa. Isoprothiolane (diisopropyl-1,3-dithiolan-2-ylidenemalonate) is used as a fungicide and an insecticide in rice paddies. We administered these agrochemicals to the male rat and examined their effects on cytochrome P450 (P450), glutathione S-transferase (GST), UDP-glucuronosyltransferase (UDPGT), and NAD(P)H-quinone oxidoreductase 1 (NQO1)-related metabolism in the liver. 2. Administration of isoprothiolane, butachlor or pretilachlor to rat induced hepatic P4502B subfamily-dependent enzyme activities (pentoxyresorufin O-depentylation and testosterone 16 beta-hydroxylation) up to 271-413% of control, which coincided with the increase in expression levels of the P4502B apoprotein. 3. Activities of GST toward 1-chloro-2,4-nitrobenzene and 3,4-dichloronitrobenzene were slightly induced (127-133% of control) in the liver of the rat treated with these pesticides. On the other hand, marked elevations of UDPGT activities toward p-nitrophenol (164-281% of control) were observed. NQO1-related metabolism (menadione reductase activity) was also induced (123-176% of control) in the liver of rat treated with these agrochemicals. 4. These results indicate that some of the agrochemicals currently in use are capable of inducing phase I and II xenobiotic-metabolizing enzyme activities in an isozyme selective manner. The induction of these activities may disrupt normal physiologic functions related to these enzymes in exposed animals.