1. The ion selectivity and gating of apamin-sensitive, small conductance Ca(2+)-activated K+ (SK) channels were studied in cultured rat adrenal chromaffin cells using patch clamp techniques. 2. The amplitude of slow tail currents showed a bell-shaped dependence on depolarization potentials. Slow tail currents were abolished in a Ca(2+)-free external solution or by adding 100 microM Cd2+ to the external solution. Reversal potentials followed the predictions of the Nernst equation for a K+ electrode. 3. Slow tail currents were largely blocked by external application of apamin (dissociation constant, Kd, 4.4 nM), (+)-tubocurarine (Kd, 20 microM), and tetraethylammonium (Kd, 5.4 mM). 4. The relative permeability (PX/PK, where X may be any one of the ions listed) of SK channels was: Tl+ (1.87) > K+ (1.0) > Rb+ (0.81) > Cs+ (0.16) > NH4+ (0.11). Na+, Li+ and methylamine were not measurably permeant (PX/PK < 0.005). Open SK channels seem to have an effective pore diameter of 0.34-0.38 nm. The relative conductance (gX/gK) was: Tl+ (1.29) > K+ (1.0) > Rb+ (0.85) > Cs+ (0.45) approximately NH4+ (0.44). 5. With mixtures of Tl+ and K+, SK channels showed anomalous mole-fraction behaviour. 6. Ca2+ dependence of SK channel gating was studied using inside-out macropatches. The [Ca2+] required for half-maximal activation and the Hill coefficient were 0.69 microM and 1.7, respectively, and independent of membrane potentials. 7. Single-channel conductance was 13-14 pS (160 mM K+).