84 SPF male rats (OFA strain) were rendered hypercapnie by breathing 8 ± 1% CO2, 21% O2, 24 h per day during 2, 4 and 8 weeks, These lots of rats (n = 12) were compared with controls (n = 12) kept under normocapnic conditions. On the last day of exposure, blood was sampled from the abdominal aorta of the anesthetized rats for hema-tologic study and electrolyte assay (K+, Na+, Ca++) in plasma and erythrocytes. Erythrocyte counts briefly increased after 2 weeks of exposure to carbon dioxide but no reticulocytosis was found. The mean cell volume decreased at 2 and 4 weeks but the calculated hematocrit remained constant. The main disturbances were a decrease in hemoglobin concentration and a decrease in the Wintrobe indexes. Leukocyte counts increased during hypercapnia with a slight reduction in neutrophils. Leukocyte peroxidase variations may be explained by a relative variation in the ratio of ‘old’ to ‘young’ granulocytes. The anorexic effect of carbon dioxide (body weight retardation) was consistently observed. The ventilatory acidosis remained uncompensated in the plasma and erythrocytes (control pHi = 7.25, hypercapnia = 7.19, 7.21 and 7.18 at the different durations). Erythrocyte K decreased at 4 and 8 weeks. Unfortunately, erratic values were found in controls at 2 weeks. The correlative increase of plasma K was not significant but the Ke/Ki ratio demonstrated an erythrocyte K shift. Na variations were nonsignificant and Ca variations were discussed in regard to metabolism, bone disturbances and in regard to the anorexic effect of carbon dioxide. The authors discuss the probable disturbances of the oxygen status in hypercapnie normoxic rats in relation to acid-base disturbances (acidosis, Pao2 increase), hematologic changes (hemoglobin and mean cellular volume decrease) and electrolyte modification in the erythrocytes (Na + K/Hb ratio).