The tectorial membrane is a gelatinous structure overlying the sensory cells in cochlea. Both the strategic location and the internal structure of this membrane suggest that it plays a key role in delivering sound stimuli to the hair cells, but exactly how the membrane performs this role is not clear. To investigate the mechanics of the tectorial membrane, interferometric measurements of the tectorial membrane's responses to sound have been made in the apical turn of the guinea-pigs cochlea in vivo and post mortem. A threedimensional (3-D) reconstruction technique was developed using an interferometer system combined with a two-axis goniometer, which detects vibrations in three orthogonal directions by using different viewing angles. Gold-coated polystyrene microbeads firmly attached to the tectorial membrane were found to move along nearly straight lines in three-dimensional space both in vivo and post mortem. The transverse motion of the observed points was nearly perpendicular to the reticular lamina and the radial motion is along the direction of the fibers of the tectorial membrane. The tectorial membrane responds to sound in a similar way to other structures in the cochlear partition. There is no obvious evidence to support the idea that the tectorial membrane resonates to form a second filter inside the cochlea. Experimental measurements show that opening of the cochlea may cause several effects to the cochlear mechanics. Rupturing of the Reissner's membrane results in a loss of strong baseline position shifts and compressive nonlinearity in sound-evoked response of the cochlear partition. However, the basic tuning properties of the partition remain unchanged