In 24 healthy adult patients having orthopedic surgical procedures requiring the use of a tourniquet under general anesthesia with controlled mechanieal ventilation, we have deter- mined ehanges in end tidal CO2(PetCO2) and arterial blood gas values before and after release of tourniquet. After deflation of tourniguet, PETCO and PaCO2 increased significantly with the maximal elevation occuring within two minutes. The pH level decreased significantly and maximally within three minutes. There was statistically significant linear correlation between PCO and PaCO2 Sugesting prediction of the PaCO2, level by monitoring the PetCO2 level. On these findings, hyperventilation may be indicated to facilitate the return of PaCO2 and pH to baseline just before and for several minutes after tourniquet release, especially in patients with increased intracranial pressure. In conclusion, we recommend noninvasive monitoring of the PetCO2 level instead of invasive measure-ment of the PaCO2 level.