The purpose of this study was to assess the effect of chronic inhalation exposure to mixtures of nitrous-oxide (10024972) (NO) and halothane (151677) (HT) on the reticuloendothelial malignancies in rats. Fischer-344-rats were exposed to 50 parts per million (ppm) NO and 1ppm HT (Group 2), or 500ppm NO and 10ppm HT (Group 3), for 7 hours per day, 5 days per week, for 104 weeks. Each group contained 100 rats. The body weight increase of study animals was similar to that of controls. There was no evidence of exposure related effects on appearance and behavior of animals. Tissue masses and wartlike lesions were observed in the ears, nose, axilla, back, and legs of experimental and control rats. The survival among the males was 74 percent for control, 76 percent for Group 2, and 72 percent for Group 3. The corresponding survival for females was 86, 78, and 84 percent, respectively. All hematological tests were within the normal limits. No exposure related lesions were apparent. A variety of non neoplastic lesions were observed in all groups. Monocytic leukemia was the most common neoplasm in all three groups, occurring in 13.8 percent of the males and 9.7 percent of the females. A large variety of tumors was observed in all three groups, which were found to be characteristic for this strain. It was concluded that long term inhalation of NO and HT mixtures did not increase the incidence of neoplasms.