Single outer hair cells from guinea pig cochlea were prepared by microdissection and cultured for several hours in a modified Hanks' medium. Cells, which were viable, judged by morphological criteria, were investigated with the patch-clamp technique. Membrane potentials up to -70 mV could be measured in the cultured cells. Ion channels were found in the basolateral membrane and in the cuticular region of the hair cells. These ion channels could play an important role in transducing sound-induced sterociliary motion into membrane depolarizations, thereby regulating the contractile state of the outer hair cells and in turn the resonance properties of the cochlear amplifier.