Volunteers will undergo arterial cannulation with a plastic 22 G catheter in a wrist artery. Standard and test (COHb measurement prototype, CMP) pulse oximeters will be attached to the fingers. Subjects will be semi-recumbent (30 degree head up) on an operating room bed and re-breathe O2 from an anesthesia machine breathing circuit to which CO will be added in amounts calculated to increase COHb by 2.5% (heavy smokers often reach 20% COHb). It is possible to control the amount of COHb formed since each milliliter of CO binds to all the hemoglobin in 5 ml of normal blood. From an estimate of the subject’s blood volume we can compute the volume of CO to be added to the rebreathing system. Arterial blood volume is approximately 8% of body weight in adult males and 7% in adult females. Arterial blood samples will be obtained after 5 minutes equilibration following each addition of CO to the system. Measurement of total Hb, HbO2, and COHb in 1 ml arterial blood samples will be made with a Radiometer ABL 90 Flex Plus. Continuous standard pulse oximetry will be performed. A multi-wavelength pulse oximeter that detects COHb will also be placed on another finger as a reference to estimate the amount of COHb present (this oximeter is FDA approved for levels below 20% COHb). Simultaneously, data from the test oximeters will be recorded. If COHb rises to above 15%, 100% O2 will be administered via face-mask until levels drop below 12%. To test the responses of the oximeters to combined hypoxemia and elevated COHb subjects will be exposed to brief periods of hypoxia (75-100% SaO2) as we have done for studies on the accuracy of pulse oximeters.