A method is presented for the insertion of microelectrodes into the scala media and fluid spaces of the organ of Corti without injury to the capillaries of the membranous labyrinth. Electrode placement is made under complete visual control and monitored on a closed‐circuit television system. Capillary blood flow and electrode position are constantly observed. With electrodes so placed, electrode current resulting from changes in oxygen concentration is measured as respiratory air to the animal is cut off and restored. Oxygen concentration in the tunnel of Corti decreases and returns to normal much more rapidly than does that of the endolymph of the scala media. These results furnish evidence that the spiral vessels of the basilar membrane (vas spirale) and spiral osseous lamina supply the oxygen to the fluids in the spaces of the organ of Corti.