1. Inner hair cells were mechanically isolated from the apical, low-frequency region of the guinea-pig cochlea and maintained by superfusion with tissue-culture medium. Membrane currents were studied under voltage clamp, using the whole-cell recording mode of the patch-clamp technique. 2. The cells were studied mostly at 35-38 degrees C to obtain realistic kinetics of the currents, relevant to the functioning of these cells in vivo. 3. Isolated inner hair cells had resting potentials of about -65 mV. Depolarizing voltage steps from a holding potential of about -80 mV resulted in large time- and voltage-dependent outward currents. Hyperpolarizing voltage steps from the same holding potential only showed a small leakage conductance of 0.5-2.5 nS. 4. On repolarization to different membrane potentials, the tail currents reversed around -75 mV. This indicates that the outward currents were mainly carried by potassium ions. 5. Pharmacological dissection of the currents provided evidence for two different potassium conductances. The largest conductance had extremely fast kinetics. Its principal time constant of activation was about 0.15-0.35 ms, the faster values being obtained for larger depolarizations. This fast potassium conductance was blocked by 25 mM-tetraethylammonium chloride in the bath. 6. A smaller, slow potassium conductance, with principal time constants of activation of 2-10 ms (speeding up with depolarization), was blocked by 10-15 mM-4-aminopyridine in the patch pipette. 7. Both potassium conductances were activated over the membrane potential range of about -60 to -20 mV. This is approximately the same as the range of the receptor potential measured in vivo. Therefore these conductances should influence the properties of the receptor potential in inner hair cells. 8. Current injection experiments showed two main effects of the potassium conductances: (a) a non-linearity in the voltage-current relationships; (b) a strongly damped oscillation of the membrane potential in response to a large step of outward current. This oscillatory behaviour is caused by the fast potassium conductance.