The chronic toxicity of a combination of nitrous-oxide (10024972) (N2O) and halothane (151677) was investigated in rats. Male and female Fischer-344 rats were exposed to N2O and halothane mixtures in inhalation chambers, 7 hours/day, 5 days/week for 104 weeks. Group 1 was control group exposed only to room air, group 2 was exposed to 50 parts per million (ppm) N2O and 1ppm halothane, and group 3 was exposed to 500ppm N2O and 10ppm halothane. Hematologic indices were measured at 13, 26, 52, 78, and 104 weeks. Necropsies were conducted on rats dying during the study, and surviving rats at the end of the study. No evidence of an exposure related change in physical appearance or behavior of the rats exposed to N2O and halothane was noted. There was a 74 percent survival in group 1 at 104 weeks, 76 percent for group 2 and 72 percent for group 3 among the males. Comparable figures for females were 86 percent, 78 percent, and 84 percent, respectively. Tumors observed in both exposed and control rats had previously observed in this rat strain. An increase in the occurrence of neoplasia in general was not seen as a result of the combined exposures to N2O and halothane. There was also no association with the incidence of reticuloendothelial tumors. Monocytic leukemia occurred less frequently (13.8 percent in males and 9.7 percent in females) than had been expected based on earlier studies.