Several reports have suggested that semen quality in normal men is declining. Given the lack of consensus about the effects of diesel exhaust (DE) on the male reproductive system, we conducted various experiments. We examined the effect of the exposure of mature male mice to DE for 6 months on the male reproductive system. Daily sperm production per gram from the testes dose-dependently decreased with exposure to DE. Next, we investigated the effect of exposure of pregnant mice to DE on male gonad development at the level of mRNA expression. Expression of mRNAs for steroidogenic factor (Ad4BP/SF-1) and Müllerian inhibiting substance (MIS) decreased significantly in male fetuses when maternal mice were exposed to DE for 8 hr per day between days 2 and 13 post coitum. In addition, DE exposure during the fetal period may have some influence on the male reproductive function in newborn mice. In utero DE exposure caused testosterone levels to increase in newborn male mice at age 4 weeks. These findings indicate that exposure to DE may influence the male reproductive system. Further studies are necessary to elucidate the mechanism of the effects of DE on the male reproductive system and to define which classes of compounds are responsible for the changes in the male reproductive system.