1. Potassium currents were characterized in tall hair cells of the chick's cochlea. Outward potassium currents were found to flow through two distinct classes of channels. 2. Individual hair cells were isolated from 200 microns long segments of the apical half of the chick's cochlea. Whole-cell voltage-clamp and current-clamp recordings were made from these cells. 3. Voltage responses to injected current ranged from high-frequency (100-250 Hz) oscillations in some cells, to slowly repetitive Ca2+ action potentials or slow oscillations (5-20 Hz) in others. 4. Ionic currents recorded in voltage clamp also varied in different hair cells. Cells with high-frequency voltage oscillations had rapidly activating Ca2(+)-dependent outward K+ current, IK(Ca). Cells that generated action potentials had slow delayed rectifier outward K+ current, IK, and inward rectifier current, IIR. All hair cells had inward Ca2+ current. 5. IK(Ca) activated positive to -45 mV. Tail currents reversed at the K+ equilibrium potential. This current was eliminated in Ca2(+)-free solutions, or when exposed to 10 mM-TEA. This outward current was fully activated within 1-3 ms at 0 mV. The whole-cell current was noisy and ensemble variance analysis suggested a single-channel conductance of 63 pS near 0 mV. 6. IK activated positive to -50 mV. Tail currents reversed at the K+ equilibrium potential. This current was not eliminated in Ca2(+)-free solutions, and was relatively resistant to external TEA. IK activated slowly, reaching peak values in 10-20 ms at 0 mV. This current showed little variance and the average single-channel conductance based on macroscopic noise near 0 mV was 8 pS. 7. External tetraethylammonium (TEA) or Ca2(+)-free saline eliminated the high-frequency voltage oscillations seen in many basal cells. In contrast TEA had little effect on slow action potentials (or low-frequency oscillations) seen in cells with IK. 8. IK(Ca) was prominent in hair cells originating 1.0-2.0 mm from the cochlear apex. IK and IIR dominated the membrane conductance of tall hair cells originating within 0.5 mm of the cochlear apex. 9. The frequency of voltage oscillation in apical cells was temperature-dependent, nearly doubling for each 10 degrees C rise in temperature. 10. IIR activated at membrane potentials negative to -75 mV. The average time constant of activation at -100 mV was 2 ms. Tail currents reversed at the K+ equilibrium potential and did not depend on the external Na+ concentration. IIR was blocked by 5 mM-Cs+ or 100 microM-Ba2+ in the external saline.