1. The gating property of a mechano-electrical transduction (m-e.t.) was studied in dissociated chick vestibular hair cells having hair bundles of varying length (from 7 to more than 30 microns long). The whole-cell recording voltage-clamp technique was used to record m-e.t. currents, and mechanical stimuli were applied to the hair bundle with a rigid glass rod. 2. Displacements of the glass rod and the hair bundle were measured with a resolution of 0.1 micron from contrast-enhanced television images. The motion of the hair bundle was tightly coupled to the motion of the stimulating glass rod, and displacements were not detected in the circumference of the cell body. Thus, all the displacement applied to the hair bundle resulted in bending about the insertion into the cuticle and relative to the cell body. 3. Displacements of the hair bundle towards the taller stereocilia generated inward-going m-e.t. currents at negative membrane potentials, while displacements of the hair bundle towards the shorter stereocilia generated outward-going m-e.t. currents. These outward going m-e.t. currents reflect closing of the m-e.t. channels which are open at the resting position of the hair bundle. The fraction of these channels open at the resting position was 0.12 +/- 0.04 (n = 7). 4. The displacement-response relationship measured both at -50 mV and at +38 mV were superimposable after scaling. Thus, no voltage dependence was observed in gating of the m-e.t. conductance. 5. When a hair bundle of a shorter length (less than 7.5 microns long) was stimulated at 5 micron form the insertion to the cuticle, the minimum hair-bundle displacement which could generate a detectable amount of m-e.t. current was 0.01 micron. The transduction current was linearly related to the hair-bundle displacement for values of up to 0.6 micron towards the taller stereocilia, and showed saturation with larger displacements. 6. When a hair bundle of a longer length (more than 12.5 microns long) was stimulated (towards the taller stereocilia) at 10 micron from the insertion to the cuticle the m-e.t. current generated was linear for displacements of up to 1.5 micron, and saturated with larger displacements. 7. The above two points suggest that the range of linear transduction becomes wider as the length of the hair bundle becomes longer under in situ conditions where the displacement is likely to be applied at the tip of the hair bundle.(ABSTRACT TRUNCATED AT 400 WORDS)