1. Deflections of the mechanosensory hair bundles on frog saccular hair cells were measured interferometrically, with submillisecond temporal and submicrometer spatial resolution, and with subnanometer displacement sensitivity. 2. The direction of the initial bundle deflection (toward the taller stereocilia) in response to a sudden application of aminoglycoside antibiotics shows that the mechanosensory channels are blocked in their mechanically open state. 3. The magnitude of the initial deflection is consistent with published data on the gating swing as derived from the gating compliance. 4. A delayed relaxation and frequently a reversal of the initial deflection were observed and are attributed to the previously reported mechanical adaptation mechanism, which is at least partially controlled by the influx of Ca2+ through the transduction channels. 5. Increases of low-frequency spontaneous motion were found at intermediate blocker concentrations. They can be well accounted for by the fluctuating force exerted on the bundle by the random binding and unbinding of blocker molecules. 6. The mechanical response of the hair bundle to aminoglycosides may be related to their acute and specific ototoxicity.