Vibration of Reissner's membrane and reticular lamina were measured at a set of radial positions in the apical turn of a living guinea pig cochlea, in response to sound applied to the ear canal. The cochlea was sealed and the Reissner's membrane was intact. A slit confocal microscope was used to identify the measurement site and confocal heterodyne interferometer was used for measuring the amplitude and phase of vibration. The X, Y, and Z coordinates of each measurement site were recorded. Using the experiment data, the motion of Reissner's membrane and the reticular lamina was animated. This animation allows us to study the relative motion of the key cochlear structures along the cochlear cross section. At the reticular lamina the amplitude of the vibration increases with distance radially from the osseous spiral lamina, reaching a maximum at the Hensen's cells. Except near the two end points, where it is attached, radial phase differences were small. The Reissner's membrane vibration amplitude is high. Phase changes rapidly with radial position, therefore different portions of the membrane do not vibrate together. The motion of the Reissner's membrane and the reticular lamina is dramatically different in amplitude and phase at each radial position. This is in contrast to the accepted concept that the motion of the two structures is the same, and raises questions as to the reason for the differences.