There are numerous experimental reports on the effects of the inhalation of CO₂ in animals but little known in the human being except the accidental inhalations. The authors experienced recently accidental inhalations of CO₂ instead of nitrous oxide for the purpose of giving nitrous oxide-oxygen-halothane anesthesia in two cases by changing color of gas cylinder. Although outcomes of above two cases are uneventful, we noted cardiac arrhythmias, oozing in the operative field, cyanotic skin, hypertension, hot canister and .easy restoration of spontaneous respiration. We estimated experimentally FiCO₂ by measuring FiO₂ with Beckman D-2 paramagnetic oxygen analyzer and reviewed literatures in discussion.