Mechanical vibrations were measured at the apical turn in living guinea pig cochlea, in response to sinusoidal acoustic stimuli, using heterodyne interferometry. The cochlea was sealed and the vibrations were measured at different cellular locations along a radial track at the level of reticular lamina and one point on the osseous spiral lamina. Averaged time waveforms were recorded at each test frequency. The nonlinearity in the apical turn is demonstrated by the distortion in the time waveforms and the richness of the harmonic components in their Fourier transforms. Tuning curves and input/output curves for the fundamental and harmonics components are shown. The fundamental component is essentially linear below about 90 dB SPL while the harmonics display strong nonlinearity and saturation. Negative feedback in the apical turn of the cochlea linearizes the response at the fundamental frequency.