An important function of the peripheral auditory system is the resolution of complex sounds into their constituent frequency components. It is well established that each mechanoreceptive hair cell of the cochlea is maximally sensitive to a particular frequency of sound, but the mechanisms by which this sharp frequency selectivity is achieved are still controversial. The complex mechanical and hydrodynamic properties of the receptor organs and of the hair cells themselves are certainly involved. However, in at least one auditory organ, the turtle cochlea, frequency tuning is greatly enhanced by the electrical properties of the hair-cell membrane; each cell in this organ behaves as an electrical resonator tuned to a narrow band of frequencies. Using the 'Gigaseal', whole-cell recording technique, we have investigated the biophysical basis of similar resonant behaviour in enzymatically isolated hair cells from the bullfrog sacculus. We report here the identification of three voltage- and ion-dependent conductances which may contribute to the electrical tuning mechanism: a non-inactivating calcium conductance, an A-type K+ conductance, and a Ca2+-activated K+ conductance.