1. The activation of calcium (Ca2+)-dependent potassium (K+) currents in dissociated rat adrenal chromaffin cells was investigated using the dialysed cell recording technique. 2. Ca(2+)-dependent K+ current was the major component of outward current at command potentials from -30 mV to about +50 mV. 3. Two components of Ca(2+)-dependent outward current could be distinguished based on the voltage dependence of activation, the properties of tail currents following repolarization, and pharmacological properties. 4. One Ca(2+)-dependent current was similar to an after-hyperpolarization current (often termed IAHP) observed in other cell types. This current was largely blocked by 200 nM-apamin or 200 microM-curare, was associated with slow Ca(2+)-dependent tail current, and exhibited little dependence on voltage. In cells with cytosolic Ca2+ buffered to 500 nM-1 microM, curare-sensitive current accounted for most of the membrane current at potentials negative to about -40 mV. 5. A second component of Ca(2+)-activated K+ current exhibited voltage-dependent activation, was completely blocked by 1 mM-TEA, and turned off rapidly following repolarization. An unusual aspect of the TEA-sensitive currents was that they appeared to inactivate under conditions of constant cytosolic Ca2+. 6. A novel observation during these experiments was a slow hump of outward current which appears to result from a non-monotonic elevation in cytosolic Ca2+ during prolonged voltage jumps.