An epoxide hydrolase (mEH) in liver microsomes was purified to apparent homogeneity from a dog treated with phenobarbital. The purified enzyme had a minimum molecular weight of 47,000 as determined by SDS-PAGE. The dog mEH activity was characterized by use of a substrate, 7-glycidoxycoumarin (GOC), and some effectors of this enzyme. In vitro activators, metyrapone, and isoquinoline, stimulated the microsomal activity, but the former had no such effect on the purified enzyme in case of this substrate. All mEH inhibitors, 1,1,1-trichloropropene 2,3-oxide (TCPO), cyclohexene oxide, and 2-bromo-4'-nitroacetophenone (BrNAP), suppressed hydrolase activity. The NH2-terminal amino acid sequence of the purified enzyme was highly homologous (90%) to the sequences deduced from a cDNA clone of rat enzyme. Antiserum to the purified enzyme raised in rabbits cross-reacted with rat and guinea pig epoxide hydrolases. No gender-difference in this enzyme in liver microsomes was observed in dogs.