A heterodyne laser interferometer was used to observe the sound-evoked displacement patterns of Reissner's membrane and various other structures in the apical turn of the guinea-pig cochlea. Most structures (including the basilar membrane) were similarly tuned, and had best frequencies in the 200-350Hz range. A distinct notch was usually observed approximately 0.7 octaves above the best frequency, and amplitude- and phase-plateaus were observed at higher frequencies. In most other respects, however, the mechanical tuning resembled the frequency-threshold curves of low frequency cochlear nerve fibers. In five reasonably intact, in vivo preparations, the frequency of the mechanical sensitivity notch was intensity-dependent: Compressive nonlinearities were observed above approximately 80 dB SPL on the low-frequency side of the notch, with antagonistically expansive nonlinearities on the high-frequency side. Two-tone suppression was observed in one of these preparations. Stimulus-related baseline position shifts were observed in another in vivo preparation. No such nonlinearities were observed in structurally damaged and/or > 1 hour post-mortem preparations. However, more robust nonlinearities were observed in all preparations at higher levels of stimulation (e.g. > 100-110 dB SPL). These high-level nonlinearities diminished only slowly after death, and gave rise to various effects, including time-dependent (i.e. adapting) and severely distorted (e.g. peak-split and/or dc-shifted) responses.