Guinea pig outer hair cells (OHCs) isolated from the two apical turns of the cochlea and firmly attached to a suction pipette, were subjected to the stimulus of the near-field (particle) displacements of a calibrated oscillating fluid jet aimed at the lateral cell walls. The longitudinal length changes of the OHCs in response to stimulation, in a direction orthogonal to that of the fluid jet, were recorded by a photodiode array. The response had two components; a phasic length change which followed the frequency of the particle displacement of the jet cycle by cycle, and a tonic length change which took several milliseconds to develop depending on the magnitude of the mechanical stimulus. When the hair cell changed length the lateral walls of the OHC moved in antiphase, moving apart during shortening and together during lengthening. With increased stimulus level the phasic response grew in proportion to the stimulus magnitude and began to saturate at the highest stimulus levels, while the tonic response grew in proportion to the square of the stimulus magnitude. Both the direction of the tonic length change and the phase of the phasic component could alter with the level of stimulation. Isolevel and isoresponse-frequency functions of both the tonic and phasic length changes revealed that both response types were tuned to similar resonant frequencies (RF) between 150 and 2500 Hz. The phase of the phasic length change began to lag at frequencies just below the RF, lagged by about 90 degrees at the RF and lagged by a further 90 degrees at frequencies above RF. The frequency response properties of the OHCs closely corresponded to those of a damped, forced, mechanical resonance. The tonic response disappeared and the phasic response was reduced at low-levels as a consequence of intense mechanical stimulation and with time.