The elimination rate of COHb in the blood was measured by a tonometer and the COHb saturation of the human blood under the exposure of 0.3% CO in the air for 30 minutes was estimated to be about 47%. The CO elimination of the tonometered blood by the exposure to 4 kinds of gases, 1 ATA air, 1 ATA O₂, 1.9 ATA O₂ and 2.8 ATA O₂, showed a linear decrease on the semi-logarithmic figure, and the gradient coefficient (k) of the straight lines showed a good reproducibility and significant difference from each other. The COHb elimination after supplying a 0.3% CO air mixture for 20 minutes tO₂3 mongrel male dogs weighing 12 to 18 kg were observed by letting them breathe 1 ATA air for 300 minutes, 1 ATA O₂ for 120 minutes, 1.9 ATA O₂ for 60 minutes and 2.8 ATA O₂ for 30 minutes. The COHb decrease of the CO-poisoned dogs showed also a semi-logarithmically linear tendency and the gradient coefficient (k) among these four gas-breathing groups showed a significant difference. The mean half-clearance time (t/2) of each group was: 1 ATA air for 188 minutes, 1 ATA O₂ for 32 minutes, 1.9 ATA O₂ for 25 minutes and 2.8 ATA O₂ for 12 minutes. The higher the O₂ partial pressure in the inspired gas increased, the greater became the decrease of the COHb clearance time and the greater became the reducting of the half-clearance time. The CO elimination by the dogs at a high PO₂, on the contrary, an opposite relation was observed. This fact may be explained by the improvement of the ventilatory and circulatory functions of the alveoli concerned with the CO elimination at a high PO₂, but at a low PO₂ the alveolar functions are impaired by CO and the O₂ supply is insufficient to cover the dysfunction, so the CO elimination is larger in the tonometered blood than in the dog blood. The procedure of OHP for CO poisoning was designed by the CO elimination rate in the dog, assuming it to be the same as that in the human, for the patients of three levels of severity of CO poisoning: Phase I is breathing of 2.8 ATA O₂ for 20 minutes, usually for the patients of each level of severity, then Phase II (breathing of 1.9 ATA O₂) follows after that for severe poisoning (COHb in the blood, 80%) for 57 minutes, for medium poisoning (COHb, 60%) for 46 minutes and for light poisoning (COHb, 40%) for 32 minutes. After the treatment of Phase II, the blood COHb content decreases to 5%, a safety level for CO poisoning. This process is practical and effective for the OHP treatment of CO poisoning, and besides it is not only safe against O₂ poisoning but also is capable of supplying rapidly sufficient amount of oxygen to improve the condition of the anoxic tissues.