The effects of radiation exposure in conjunction with oltipraz, a chemopreventive agent, on the expression of the gene encoding hepatic microsomal epoxide hydrolase (mEH) were examined in rats. Rats exposed to a single dose of 3 Gy gamma rays exhibited timerelated changes in the hepatic mEH mRNA level. Whereas the mEH mRNA level was transiently decreased at 3 and 8 h after irradiation, the mRNA levels were increased 3- to 4-fold at 15 to 48 h postirradiation, returning to the level in untreated animals at 72 h. Treatment of rats with oltipraz resulted in 1- to 19-fold increases in hepatic mEH mRNA levels 24 h post-treatment at doses of 5-200 mg/kg. Although treatment with oltipraz at a dose of 30 mg/kg affected the mEH mRNA level minimally (i.e. approximately 2-fold), 3 Gy whole-body irradiation along with oltipraz treatment resulted in a 9-fold increase in the mEH mRNA level at 24 h post-treatment. Treatment of animals with both oltipraz and 3 Gy gamma radiation for 3 consecutive days resulted in a 7-fold increase in mEH mRNA, showing that the increases in mEH mRNA were enhanced by the combination treatment. In rats irradiated with 3 Gy for 5 consecutive days, however, the mEH mRNA level failed to increase due to cell injury. Studies were further designed to assess the effects of 0.5 Gy ionizing radiation and concomitant oltipraz treatment. RNA blot analysis showed that mEH mRNA levels failed to be significantly altered at 3, 8, 15, 24 and 48 h after a single dose of 0.5 Gy. Nonetheless, exposure of animals to 0.5 Gy daily for 3 to 5 consecutive days caused a 3-fold elevation in the hepatic mEH mRNA level. Furthermore, treatment of animals with both oltipraz (30 mg/kg/day) and 0.5 Gy of gamma rays resulted in an enhanced elevation in the mEH mRNA level at 24 h post-treatment compared to the individual treatment, resulting in a 7-fold relative increase. The enhanced expression of hepatic mEH mRNA by 0.5 Gy gamma radiation and oltipraz was also observed after treatment for 3 to 5 days (8- to 6-fold relative increases). Western immunoblot analyses showed that hepatic microsomes produced from the rats treated with 0.5 Gy daily for 3 to 5 days resulted in a approximately 2-fold induction of hepatic mEH and that rats exposed to radiation in combination with oltipraz showed 3-fold increases in the liver mEH protein. Thus the relative increase in mEH mRNA levels was consistent with the expression of the protein. These results demonstrate that ionizing radiation causes alterations in hepatic mEH gene expression with the induction of the protein and that the mEH gene expression is enhanced by oltipraz treatment.