Blood oxygenation in cerebral vessels is an essential parameter to evaluate brain function and to investigate the coupling between local blood flow and neuronal activity. We apply resonance Raman spectroscopy in vivo to study hemoglobin oxygenation in cortex vessels of anesthetized ventilated mice. We demonstrate that the pairs of Raman peaks at 1355 and1375 cm-1(symmetric vibrations of pyrrol half-rings in the heme molecule), 1552 and 1585 cm-1and 1602 and 1638 cm-1(vibrations of methine bridges in heme molecule) are reliable markers for quantitative estimation of the relative amount of oxyhemoglobin in venules, arterioles, and capillaries. In vivo measurements of blood oxygenation in the cortex of mice ventilated with inspiratory gas mixtures containing different amounts of oxygen - normoxia, hyperoxia and hypoxia - validate the proposed approach. Our method allows to visualize blood saturation with O2in different microvascular networks.