AbstractCyclohexene oxide (CCHO) and 1,1,1‐trichloropropene‐2,3‐oxide (TCPO) are inhibitors of epoxide hydratase and have been used in studies of the mechanisms of mutagenesis in bacterial mutagenesis assays. The present studies were designed to investigate the mutagenic activity of CCHO and TCPO in Salmonella typhimurium employing the Ames histidine‐reversion assays (TA98, TA100, TA1535, TA1537, TA1538) and a forward mutation assay that uses 8‐azaguanine resistance in TM677 as the genetic marker. In the reverse mutation assay, TCPO (10−3M) produced a mutagenic response in strains TA100 and TA1535 in the absence or presence of a rat liver metabolizing system (S9), indicating that TCPO causes base‐pair substitution mutations. CCHO (10−3M) showed a slight but significant mutagenic effect in strain TA100, with or without S9 and, in strain TA1535, only in the absence of S9. In the forward assay, TCPO was a strong mutagen at concentrations above 5 × 10−5M and was toxic to the bacteria. The mutagenic and toxic effects of TCPO were slightly reduced in the presence of the S9 preparation, suggesting that the epoxide may be metabolized by the microsomal enzymes. In the forward assay, CCHO showed no mutagenic activity but some toxicity at 3 × 10−3M. When epoxide hydratase activity was measured under the conditions of the forward mutation assay, 85% inhibition of activity was observed at 10−3M TCPO, a concentration that caused a 45‐fold increase in the mutation frequency. CCHO (3 × 10−3M) produced a 55% inhibition of epoxide hydratase activity without exhibiting mutagenic effects in TM677. These results indicate that CCHO should be employed in preference to TCPO when inhibition of epoxide hydratase activity is required in b