Male mice were exposed to ethylene oxide(Et0) at concentrations of 54, 360 mg/m3 for two hours a day, on six days a week for 14 weeks. The results of sperm abnormalities test in mice showed the mutation frequencies of both low (54 mg/m3) and high (360 mg/m3) concentration groups are abvious (They are 29.40±16.24, P<0.05 and 52.91±11.97, P<0.001, respectively.). These showed that it causes evident mutagenesis to inhale ethylene oxide for a long time although in the low concentration. In light microscopic observation of the testis, the numbers of germ cells and layers of seminiferous epithelium decreased slightly in 54 mg/m3 group, but obviously in 360 mg/m3 group. Sometimes multinucleated giant cells were observed in the damaged seminiferous tubules, and more changes could be seen in the examination of seminiferous tubules under an electron microscope. The changes are: acrosomes of a lot of spermatid during spermiogenesis developmented teratoid and have not had a marked deviation of the hooked acrosome from the long axis of the nucleus, which is a charater of a sperm in mice; excessive nucleoplasm of sperm is eliminated incompletly; autolysis in a lot of spermatid was observed in development; mitochondria which located around the tails of sperm decreased and even disappeared.rn These abnormal ultrastructures on morphology in spermatogenesis associated the results of Sperm Abnormalities Tests, Dominat-Lethal Tests and Heritable Translocation Tests and so on, which are studied in cytogenetic, explained that Eto is a potent antispermatogenic agent and a teratogen. Indeed, the possiblility of inferti-lity and the teratogenesis in offspring among male workers potentially exposed to Eto should be studied further.