Intracellular photorelease of Ca 2+ from caged Ca 2+ (DM-nitrophen or nitr5) and the patch-clamp technique in the whole-cell configuration were used to investigate Ca2+-activated currents in inner hair cells (IHCs) of the mammalian cochlea. Photoliberation of intracellular Ca 2+ activated outward currents with a mean amplitude of 260 + 110 pA when IHCs were voltage-clamped, near the resting membrane potential, at -50 mV. The photoactivated currents were reversibly blocked by extracellular application of tetraethylammonium (TEA, 10mM), neomycin (1 mM) and charybdotoxin (1 gM), but not by apamin. The voltage dependence of membrane currents activated by photolysis of DM-nitrophen demonstrated a reversal potential near the K + equilibrium potential (Ek) and saturation near 0 mV. The presence of Ca2+-activated currents was further confirmed by the effects of extracellular adenosine 5'-triphosphate (ATP, 10 gM) and the Ca 2+ ionophore ionomycin (10 gM). Both agents raised intracellular Ca 2+ and simultaneously activated outward currents when IHCs were voltage-clamped near the resting membrane potential. In experiments where currents were activated by depolarizing voltage steps, nifedipine (50 gM) and Cd 2~ (1 raM) reduced significantly (20-50 %) the whole-cell outward currents, suggesting the presence of L-type Ca 2+ currents activating K § currents. These results are the first direct evidence for Ca2+-activated K + currents in mammalian IHCs, these currents being potentially important for cell repolarization during sound-induced depolarization and synaptic transmission.