The mechanical behaviour of the ciliary bundles of hair cells in the turtle cochlea was examined by deflecting them with flexible glass fibres of known compliance during simultaneous intracellular recording of the cell's membrane potential. Bundle motion was monitored through the attached fibre partially occluding a light beam incident on a photodiode array. The change in photocurrent was assumed to be proportional to bundle displacement. For deflexions of 1-100 nm towards the kinocilium, the stiffness of the ciliary bundles was estimated as about 6 X 10(-4) N/m, with the fibre attached to the top of the bundle. When the fibre was placed at different positions up the bundle, the stiffness decreased approximately as the inverse square of the distance from the ciliary base. This suggests that the bundles rotate about an axis close to the apical pole of the cell and have a rotational stiffness of about 2 X 10(-14) N. m/rad. Step displacements of the fixed end of the flexible fibre caused the hair cell's membrane potential to execute damped oscillations; the frequency of the oscillations in different cells ranged from 20 to 320 Hz. Displacements towards the kinocilium always produced membrane depolarization. The amplitude of the initial oscillation increased with displacements up to 100 nm and then saturated. For small displacements of a few nanometres, the hair cell's mechanoelectrical sensitivity was estimated as about 0.2 mV/nm. Force steps delivered by the flexible fibre caused the bundle position to undergo damped oscillations in synchrony with the receptor potential. The mechanical oscillations could be abolished with large depolarizing currents that attenuated the receptor potential. When placed against a bundle, a fibre's spontaneous motion increased and became quasi-sinusoidal with an amplitude several times that expected from the compliance of the system. It is suggested that the hair bundle drives the fibre. We conclude that turtle cochlear hair cells contain an active force generating mechanism.