We describe a protocol for scanning measurement of the basilar membrane vibration in sensitive gerbil cochleae. The scanning interferometer consists of a sensitive heterodyne laser interferometer, a microscope, and a computer-controlled three-dimensional positioning system. Approximately 1-mm basilar membrane is exposed through the surgically opened round window. The magnitude and phase of the basilar membrane vibration in response to a best-frequency tone are measured as functions of the longitudinal and radial locations. The volume velocity of the basilar membrane vibration centered at the best-frequency location is derived from the longitudinal and radial data. These data together with characteristic impedance of the cochlear fluid are used to quantify the power gain of the basilar membrane vibration, which is critical for studying the cochlear-amplifier mechanism.