The specific role that polymorphisms in xenobiotic metabolizing enzymes play in modulating sensitivity to 1,3-butadiene (BD) genotoxicity has been relatively unexplored. The enzyme micrasomal epoxide hydrolase (mEH) is important in detoxifying the mutagenic epoxides of BD (butadiene monoepoxide [BDO], butadiene diepoxide [BDO2]). Polymorphisms in the human mEH gene appear ta affect the function of the enzyme. We exposed mice with normal mEH activity (WT) and knockout mice without mEH activity (KO) to 20 ppm BD (inhalation) or 30 mg/kg BDO2 (intraperitoneal [IP] injection). We then compared Hprt mutant frequencies (MFs) among these groups. KO mice exposed to BD exhibited a significant (P < 0.05) 12.4-fold increase in MF over controls and a significant 5.4-fold increase in MF over exposed WT mice. Additionally, KO mice exposed to BDO2 exhibited a significant 4.5-fold increase in MF over controls and a significant 1.7-fold increase in MF over exposed WT mice. We also compared genomic damage in WT and KO mice (comet tail moment) following IP exposure to 3 mg/kg and 30 mg/kg BDO2. KO mice exposed to 3 mg/kg exhibited significantly more DNA damage than controls (7.5-12.1-fold increase) and exposed WT mice (3 mg/kg; 4.8-fold increase). KO mice exposed to 30 mg/kg BDO2 exhibited significantly more DNA damage than all other groups (2.327.9-fold increase). Correlation analysis indicated that a significant, positive relationship (r2 = 0.92) exists between comet-measured damage and Hprt MFs. The lack of mEH activity increases the genetic sensitivity of mice exposed to BD and BDO2. This model should facilitate a mechanistic understanding of the observed variation in human genetic sensitivity following exposure to BD. © 2003 WileyLiss, Inc.