The purpose of this study was to observe hematologic alterations following prolonged inhalation of the nitrous oxide-oxygen in rats. For this study, 56rats weighing an average of 150gm were selected and devided into control and experimental groups. The experimental groups were then categorized as fallows: 1. 20 percent nitrous oxide-80 percent oxygen group. 2. 50 percent nitrous oxide-50 percent oxygen group. 3. 70 percent nitrous oxide-30 percent oxygen group. The experimental groups were exposed within closed chamber to an atmosphere of above concentrations at a flow of 2L.fminute for either 8 hours (Intermittent group) or 24 hours (Continuous group) daily for various period. Carbon dioxide was absorbed by soda 1ime. Whenever the chamber was opened for addition of food and water or removal of animals the inflow rate of nitrous oxide-oxygen was increased m 10L./minute for 5 minutes, At the 1st, 3rd, 6th, and 7th days, experimental groups were sacrificed by giving Ketamine HCL of 1mg/100gm of body weight with intramuscular injection. When the animals anesthetized, the thoraces were opened and blood samples were drawn into a 5ml disposable syringe from the heart then placed in a bottle containing E.D.T.A. This blood samples were used for counts of red blood cell, white blood cell, hematocrit, hemoglobin, and used to make a smear for white blood cell differential counts and studies. These counts were immediately performed en the sacrificed time. The red blood cell and white blood cell counts were made by standard method and the differential leukocyte counts were conducted by examination of blood films stained with Wrights stain. The hemoglobin concentration was determined with Corning 950 Hemoglobinmeter (Corning Co., U.S.A.) and the hematocrit value was determined by the method of capillary hematocrit. The mean corpusclar volume (M.C.V.), mean corpuscular hemoglobin (M.C.H.), and mean corpuscular hemoglobin concentration (M.C.H.C.) were calculated by the method of Wintrobe. The following results were obtained: 1. The significant eviedence of leukopoietic depression was found on the 5th and 7th days of continuous exposure groups to 50%, 70% nitrous oxide. (0.82±0.18 -2.65±0.15 10(3)/Cu, mm., p<0.01) 2. Red blood cell counts were within normal limits. 3. Hemoglobin concentration suggested a significant decrease on the 7th day dxperimental groups. (10.5±0.2 -11.6±Omg/100ml., p<0.01) 4. Hematocrit values suggested a significant decrease on the 3rd, 5th and 7th days of continuous exposure to 70% nitrous oxide. (29.0±1.0 -31.25±0.5%, p< 0.01) 5. Mean corpuscular volume (M.C.V.) values suggested a significant decrease on the 5th and 7th days of all experimental groups. (39.6±1.29 -52.78±2.46 Cu. μ., p<0.01) 6. Mean corpuscular hemoglobin (M.C.H.) values suggested a significant decrease on the 1st, 7th days of continuous exposure groups to 20%, 50% nitrous oxide. (13.13±0.05 -16.61±0.2 μμg., p<0.01) 7. The significant evidence of neutropenia was found on the 3rd day to 20% continuous, 70% intermittent exposure groups and the 5th, 7th days all experimental groups. (0-8.75± 5.29%. p<0.01) 8. Lymphocytes were relatively resistant to nitrous oxide in the all experimental groups.