FC-43-for-blood substitution experiments were conducted in anesthetized rats to evaluate NIR spectroscopic responses by living brain to exchange transfusion at very low hematocrits. The NIR responses to the exchange process also assess the ability of multiwavelength algorithms to measure independent changes in the relative amounts of Hb, tHbO2 and oxidized cytochrome a,a3 in the tissue. These data indicate that FC-43 circulation at high FIO2 (1.00) delivers sufficient O2 to the rat brain to maintain the oxidation state of the mitochondrial oxidase near pre-exchange values. The results also confirm the ability of four wavelength algorithms to distinguish changes in the oxidized copper band of cytochrome a,a3 from changes in absorption by Hb and HbO2.