K channels of bovine adrenal chromaffin cells were studied using patch-clamp techniques. Whole-cell K currents measured near +10 mV were much larger in 1 mM-external Ca than in Ca-free saline. Noise analysis suggested that this Ca-dependent current was carried by a large unitary conductance channel, called BK channel, which was previously described in inside-out patches (Marty, 1981). The Ca-dependent K current near +10 mV declined with time due to 'run-down' of Ca channels. At the same time, a fraction of the outward current observed above +50 mV was also eliminated. This outward current component probably represents K efflux through Ca channels. Whole-cell Ca-dependent K currents were studied using various Ca buffers. EGTA buffers were surprisingly inefficient: in order to block the current entirely, it was necessary to use an isotonic EGTA solution and to increase internal pH. 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was at least five times more efficient than EGTA. In isolated patches three types of single-channel K currents were observed. Under normal ionic conditions (140 mM-K inside, 140 mM-Na outside), the unitary conductances measured between -20 and +40 mV were 96 pS, 18 pS and 8 pS. The 96 pS channels are the Ca-dependent BK channels. 18 pS and 8 pS channels were both activated and then inactivated by membrane depolarization. Both displayed complex kinetics; single-channel currents were grouped in bursts. Activation and inactivation kinetics were faster for the 18 pS channel (therefore termed FK channel, for fast K channel) than for the 8 pS channel (SK channel, for slow or small amplitude channel). The voltage dependence of opening probability was steeper for the FK channel as compared to the SK channel.