Male and female Swiss-Webster mice were exposed to either air (Group 1, n=179), 10 percent nitrous-oxide (10024972) (N2O) (Group 2, n=152), or 40 percent N2O (Group 3, n=151) for 4 hours per day, 5 days per week for 78 weeks followed by a 5 week nonexposure period. Surviving animals, and animals dying at earlier points, were subjected to complete autopsies. Mean body weights for Group 2 were similar to those in the control group, but Group 3 exhibited 5 percent lower body weights than controls after the 60th week of exposure for males and after the 40th week for females. Nonneoplastic lesions observed in the tissues were not related to treatment. There was no microscopic evidence of cellular damage nor evidence of megaloblastic changes or bone marrow depression. Most of the neoplastic lesions observed (the first appeared after 26 weeks of treatment) in control and test animals were either lung adenomas of alveolar cell origin or liver tumors of the basophilic hepatocellular adenoma type; there was no significant difference in the occurrence of tumors in control and exposed groups. Based on the results of this and other studies, the authors conclude that N2O has little or no carcinogenic potential.