An optical coherence tomography (OCT) system is built to acquire in vivo both images and vibration measurements of the organ of Corti of the guinea pig. The organ of Corti is viewed through a ~300-μm-diam hole in the bony wall of the cochlea at the scala tympani of the first cochlear turn. In imaging mode, the image is acquired as reflectance R(x,z). In vibration mode, the basilar membrane (BM) or reticular lamina (RL) are selected by the investigator interactively from the R(x,z) image. Under software control, the system moves the scanning mirrors to bring the sensing volume of the measurement to the desired membrane location. In vivo images of the organ of Corti are presented, indicating reflectance signals from the BM, RL, tectorial membrane, and Reissner's membrane. The tunnel of Corti and the inner sulcus are also visible in the images. Vibrations of ±2 and ±22 nm are recorded in the BM in response to low and high sound levels at 14 kHz above a noise floor of 0.2 nm.