Intracellular current administration evokes rapid, graded, and bidirectional mechanical responses of isolated outer hair cells from the mammalian inner ear. The cells become shorter in response to depolarizing and longer in response to hyperpolarizing currents in the synaptic end of the cell. The cells respond with either an increase or decrease in length to transcellular alternating current stimulation. The direction of the movement with transcellular stimuli appears to be frequency dependent. Iontophoretic application of acetylcholine to the synaptic end of the cell decreases its length. The microarchitecture of the organ of Corti permits length changes of outer hair cells in a manner that could significantly influence the mechanics of the cochlear partition and thereby contribute to the exquisite sensitivity of mammalian hearing.