A carcinogen bioassay of nitrous oxide (N2O) was performed in groups of male and female Swiss-Webster mice exposed to either air (n = 179), 10% N2O (n = 152), or 40% N2O (n = 151) for 4 h per day, 5 days per week. After 78 weeks of exposure, there was a 5- week period without treatment following which surviving mice were killed. Mice killed at this time or dyingin extremisat other times were subjected to complete autopsy unless advanced autolysis or cannibalism precluded examination. Mean body weights for male and female mice in the 10% N2O group were the same as those in the air control group throughout the study, whereas they were 5% less in the 40% N2O group. Mean organ weights for N2O-treated mice were not statistically different from those of control mice. Gross and microscopic examination of tissues revealed a variety of neoplastic and nonneoplastic lesions; however, their presence was unrelated to treatment.