Four groups of B6C3F1 mice, each containing 30 per sex, were exposed to ethylene oxide (EtO) vapor at target concentrations of 250, 100, 50, 10, or 0 ppm which are equivalent to 450, 180, 90, 18, and 0 mg/m3, respectively. The actual mean chamber concentrations were 236, 104, 48, 10, and 0 ppm, respectively. After an exposure regimen of 6 hr per day, 5 days per week, for 10 weeks (males), or 11 weeks (females), urine and blood samples were taken for clinical pathologic evaluation and selected tissues were weighed and examined by light microscopy. Statistically significant findings that could be indicative of a toxic response were observed in the 250-ppm exposure group. These included minimal changes in certain erythroid parameters, increased liver weight, decreased testicular weight, and decreased spleen weight which was noted also in the 100-ppm group. However, there were no microscopic findings to support or explain any of these apparently treatment-related effects. Results of a neuromuscular screening test indicated that certain reflex responses and locomotor activities were altered in the EtO-exposed animals. A dose-related trend of response in the 250-, 100-, and 50-ppm exposure groups was noted in the evaluation of locomotor function; however, because of the small sample size, it was not possible to determine a threshold concentration for this effect. There were no accompanying histopathologic alterations in muscle and central or peripheral nervous tissue.