The action of aminoglycoside antibiotics on transduction by hair cells was investigated in isolated preparations of the bullfrog's sacculus. Bath application of aminoglycosides produced a reversible blockage of extracellularly recorded responses to displacements of the otolithic membrane. The half-blocking concentrations for various drugs were in the range 2-95 microM. The effect of dihydrostreptomycin on the receptor currents of individual hair cells was studied under two-electrode, voltage-clamp conditions. Iontophoretic application of drug to the apical cellular surface caused a reduction of the receptor current within 20 ms; the reduction was reversible within 1 s. The effect was most striking at holding potentials more negative than -60 mV and was relieved by depolarization. The effect of intracellular aminoglycosides was investigated in cells voltage-clamped with the tight-seal, whole-cell technique. Gentamicin and dihydrostreptomycin, at concentrations near 100 microM, did not block transduction under these conditions. The acute, reversible blocking effect of aminoglycosides therefore occurs from the extracellular membrane surface. The results are consistent with aminoglycosides' plugging the poorly ion-selective transduction channels of hair cells.