Rapid-scanning reflectance spectrophotometry was used to evaluate the reduction-oxidation state of cytochrome oxidase in normoxic rat brain. Reflectance spectra were recorded from intact blood-perfused cerebral cortices after increased oxidative metabolic activity induced by direct cortical stimulation. Reflectance spectra taken from blood-free rat brain and from the rat ear vascular bed were used to identify cytochrome and hemoglobin components of spectra taken from intact brain. Cortical stimulation provoked shifts toward increased oxidation of cytochrome oxidase that were detectable in reflectance spectra. Observations of an oxidizable fraction of cytochrome oxidase demonstrate that a fraction of the cytochrome oxidase pool exists in a reduced state in normoxic brain. The presence of reduced cytochrome oxidase suggests that oxygen delivery to the brain is restricted by microvascular control mechanisms either as a function of brain metabolic physiology or as protection against oxygen toxicity.