The isolated lateral-line organ of Xenopus laevis was used to study reversible effects of low concentrations of aminoglycoside antibiotics on sensory hair cell functioning. Extracellular receptor potentials and single fibre afferent nerve activity were recorded during stimulation of the lateral-line organ with sinusoidal water movements. The results show that dihydrostreptomycin in concentrations between 2.5 and 17.5 micrograms/ml caused an increase in spontaneous afferent nerve activity, probably by increasing the spontaneous transmitter release at the afferent synapse. In addition, the same concentrations of dihydrostreptomycin markedly affected the mechano-electric transduction process, resulting in a phase lag in th receptor potential, the magnitude of which increased with increasing concentration up to a value of 0.4 pi radians. The effect of dihydrostreptomycin on the amplitude of the receptor potentials depended on the stimulus frequency. Other aminoglycoside antibiotics, namely streptomycin, gentamycin and kanamycin, produced similar effects but at different concentrations. It is concluded that low concentrations of aminoglycoside antibiotics have a dual action upon the sensory hair cells. They increase the spontaneous activity by affecting the hair cell membrane and at the same time they impair the mechano-electric transduction process resulting in a large phase lag in the receptor potentials. The latter effect, which is antagonized by Ca2+, may be due to interference by the antibiotics with the mode of motion of the sensory hairs.