Volunteers will undergo arterial and venous cannulation. Test prototype pulse oximeter probes will be attached to the fingers. Subjects will be in a semi-recumbent position on an operating room bed and receive sodium nitrite 2-3 mg/kg intravenously to induce methemoglobin formation. Arterial blood samples will be obtained every 5 minutes for measurement of total Hb, HbO2, and metHb with a Radiometer ABL-90 Flex hemoximeter. If methemoglobin is < 5%, an additional 2-3 mg/kg sodium nitrite will be given. Continuous measurement of oxyhemoglobin and methemoglobin with a multi-wavelength pulse oximeter will be made. Simultaneously, data from the test oximeters will be recorded. If metHb increases to above 15%, 1% methylene blue (0.5-5.0 mg/kg) will be administered intravenously to convert the metHb to normal hemoglobin. Mask O2 will also be administered if metHb increases to above 15%, otherwise subjects will breathe room air. To further test the oximeters, subjects will be exposed to brief periods of hypoxia in accordance with hypoxia lab protocols for SpO2 testing. This will establish if accuracy of metHb detection is preserved when deoxyHb increases.