The ability of a number of known inhibitors of catalase activity to affect cytosolic and microsomal epoxide hydrolase activities in vitro, measured as enzymatic trans-stilbene oxide hydrolysis and styrene oxide hydrolysis, respectively, was investigated. Catalase and cytosolic epoxide hydrolase activities are inhibited by hydroxylated metabolites of 2-amino-4,5-diphenylthiazole (DPT). The metabolite hydroxylated on the Cphenyl ring (40H-DPT) and the metabolite hydroxylated on both phenyl rings (4,s- DIOH-DPT) are potent inhibitors of both enzymes; the metabolite hydroxylated on the 5-phenyl ring (SOH-DPT) is less potent. Unmetabolized DPT has no effect on either enzyme. 40H-DPT inhibits, but 50H-DPT enhances, microsomal epoxide hydrolase activity. 4,5-DIOH-DPT and DFT have no effect on this enzyme. Other compounds that inhibit both catalase and cytosolic epoxide hydrolase activities, but do not inhibit microsomal epoxide hydrolase activity, are nordihydroguaiaretic acid and 2-aminothiazole. Microsomal epoxide hydrolase activity is enhanced by 2-aminothiazole and levamisole in vitro. Thus these inhibitors of catalase are selective epoxide hydrolase inhibitors in that they inhibit cytosolic epoxide hydrolase activity in vitro, but have either no effect on, or increase the activity of, microsomal epoxide hydrolase in vitro. Conversely, the selective cytosolic epoxide hydrolase inhibitors lphenylchalcone oxide and 4'-phenylchalcone oxide do not inhibit catalase activity, nor does trichloropropene oxide, a selective microsomal epoxide hydrolase inhibitor.