1. Whole-cell and single-channel recordings were used to characterize Ca(2+)-activated K+ channels (IK(Ca)) in acutely dissociated chick-ganglion neurones. 2. Application of depolarizing voltage steps resulted in outward currents that could be separated according to their dependence on external Ca2+ and/or holding potential. IK(Ca) was the only outward current that could be evoked from holding potentials of -50 mV or less. IK(Ca) was eliminated by bath application of Ca(2+)-free salines. A voltage-dependent outward current (IK(V)) could be evoked from more negative holding potentials in Ca(2+)-free salines. IK(V) was only partially blocked by as much as 30 mM-tetraethylammonium (TEA). 3. Tail currents associated with IK(Ca) reversed close to the K+ equilibrium potential (EK). IK(Ca) tail currents appeared sigmoidal, but the falling phase of the tail currents could be fitted with exponential curves that decayed faster at more negative membrane potentials. 4. IK(Ca) was blocked completely and reversibly by 10 mM-TEA. IK(Ca) was substantially reduced (80-90%) by as little as 1 mM-TEA. 5. Total IK(Ca) was reduced but not eliminated by saturating concentrations of apamin (200 nM). This blockade was not reversible with up to 30 min of washing. Application of 100 microM-d-tubocurare (dTC) also produced a partial blockade of total IK(Ca). 6. Whole-cell current-clamp recordings showed that IK(Ca) contributed to the late phases of spike repolarization and was the dominant current flowing during the spike after-hyperpolarization (AHP). Application of 200 nM-apamin caused a reduction in the duration of the AHP. This reduction was best seen when multiple spikes were evoked by prolonged (20-50 ms) injections of depolarizing current. 7. Three distinct types of IK(Ca) channels could be observed in inside-out patches in the presence of free Ca2+ concentrations of 2 x 10(-7) M, but not in the presence of free Ca2+ at concentrations of less than 10(-9) M. These had unitary chord conductances of 190 pS (i1), 110 pS (i2), and 45 pS (i3) with [K+]o = 150 mM and [K+]i = 75 mM. Each of these three channels had distinct kinetic properties. The 45 pS channel was most sensitive to activation by Ca2+ and could be detected at free Ca2+ concentrations as low as 10(-8) M. 8. All three IK(Ca) channels could be observed in inside-out patches held at membrane potentials where IK(V) was fully inactivated. Application of 10 mM-TEA caused a complete block of IK(Ca) channels in outside-out patches.(ABSTRACT TRUNCATED AT 400 WORDS)