Cell-attached and cell-free configurations of the patch-clamp technique were used to investigate the conductive properties and regulation of the major K⁺ channels in the basolateral membrane of outer hair cells freshly isolated from the guinea pig cochlea. There were two major voltage-dependent K⁺ channels. A Ca²⁺-activated K⁺ channel with a high conductance (220 pS, P_K/P_Na = 8) was found in almost 20% of the patches. The inside-out activity of the channel was increased by depolarizations above 0 mV and increasing the intracellular Ca²⁺ concentration. External ATP or adenosine did not alter the cell-attached activity of the channel. The open probability of the excised channel remained stable for several minutes without rundown and was not altered by the catalytic subunit of protein kinase A (PKA) applied internally. The most frequent K⁺ channel had a low conductance and a small outward rectification in symmetrical K⁺ conditions (10 pS for inward currents and 20 pS for outward currents, P_K/P_Na = 28). It was found significantly more frequently in cell-attached and inside-out patches when the pipette contained 100 µM acetylcholine. It was not sensitive to internal Ca²⁺, was inhibited by 4-aminopyridine, was activated by depolarization above 30 mV, and exhibited a rundown after excision. It also had a slow inactivation on ensemble-averaged sweeps in response to depolarizing pulses. The cell-attached activity of the channel was increased when adenosine was superfused outside the pipette. This effect also occurred with permeant analogs of cAMP and internally applied catalytic subunit of PKA. Both channels could control the cell membrane voltage of outer hair cells.