The genotoxic effect of maternal exposure to diesel engine exhaust was studied by analyses of sister chromatid exchanges (SCEs) and formation of micronuclei with fetal liver cells. Fetal liver cells were obtained from pregnant ICR/JCl mice at the 16th day of gestation. Diesel engine exhaust was generated by running a small engine (YANMAR NSA-40CE, used as an electric generator, displacement: 269 cc). The concentrations of pollutants in the inhalation chamber were as follows; CO: 40-100 ppm, NO2: 2-4 ppm, and particulates: 2-4 mg/m3. Maternal mice were divided into three groups: the long-term exposure group, the short-term exposure group and the non-exposed control. Mice of the long-term group were exposed to the exhaust 4 hours daily from day 0 to the 16th day of gestation. Mice of the short-term group were similarly exposed but only on the 15th and 16th days of gestation. The number of SCEs of fetal liver cells was significantly increased in both the exposed groups. The long-term exposure group showed a significant increase in the number of SCEs in comparison with that of the short-term exposure group. In the micronucleus test with fetal liver cells, no significant change in the frequency of micronucleated cells was induced by exposure to diesel exhaust.