Voltage-clamp recordings were done on hair cells from a region of the chick's cochlea. In the adult, these cells have voltage-sensitive Ca currents and rapid, Ca-activated K currents that together support an electrical resonance, showing voltage oscillations at frequencies greater than 100 Hz. In embryos 14-days old (at one week before hatching) the same cells had a voltage-sensitive Ca current like that in adults, but a more slowly acting K current (of the delayed-rectifier type). In current-clamp they could generate only slowly repetitive action potentials. By two days before hatching, Ca-activated K currents were present. We suggest that the acquisition of Ca-activated K currents contributes to functional maturation of the chick's cochlea.