We evaluated measurements of oxygen saturation from four noninvasive pulse oximeters, and two conventional arterial oximeters. Simultaneous measurements were obtained on each instrument on three different occasions in five healthy subjects breathing gas with an FIO2 of 1.00, 0.50, 0.21, 0.17, 0.15, 0.13 and 0.11. Mean bias relative to the sum of oxy-, carboxy-, and methemoglobins ranged from -0.4 to -2.6 percent for the pulse oximeters and +1.8 to -4.2 percent for the conventional oximeters. Two pulse oximeters performed well at all saturations down to 65 percent, while the others showed either increasing or decreasing bias below 80 percent saturation. Precision was approximately 2 percent for all instruments except one conventional oximeter with a precision of 0.7 percent. In the clinically relevant range, the performance of the noninvasive pulse oximeters was similar to conventional oximeters using arterial samples.