A noninvasive spectrophotometric technique for the measurement of oxygen saturation of the blood in discrete retinal vessels is described. The instrument, the retinal vessel oximeter, uses scanning fundus reflectometry to determine the optical density of a retinal vessel at three wavelengths (558, 569, and 686 nm). Oxygen saturation is determined after compensation for the effects of light scattering by the red blood cells by relating the measured densities with the corresponding extinction coefficients nf oxyhemoglobin and deoxy-genated hemoglobin. The vessel diameter is also measured continuously. All data acquisition and analysis are performed on-line by means of a microcomputer, and a vessel tracking system is used to compensate for the effects of eye movements. Oxygen saturation measurements for blood flowing through glass capillaries are presented as well as representative results of oxygen saturation measurements on normal human subjects.