Previous studies have indicated that a 16-pS K⁺ channel (KC_ca) in the basolateral membrane is responsible for the acetylcholine-induced whole-cell K⁺ conductance in these cells. In the present study we have examined this channel in excised inside-out patches of the basolateral membrane. Over a wide voltage range this channel showed inward rectification. The Ca²⁺ sensitivity was very marked, with a Hill coefficient of three and with half-maximal activation at 330 nmol/l. After several minutes most channels showed a slow run-down. Channel activity could be refreshed by addition of ATP (1 mmol/l) to the bath solution. The non-metabolizable derivative 5’-adenylylimidodiphosphate (AMP-PNP) had no such effect. In contrast, it inhibited channel activity by some 50%. ATP and its derivatives had no effect on the Ca²⁺ sensitivity. Channels activated by ATP were subsequently studied in the presence of alkaline (10 kU/l) or acidic (1 kU/l) phosphatase. Both phosphatases reduced channel activity significantly. These data suggest that the 16-pS K⁺ channel is directly controlled by cytosolic Ca²⁺. This regulatory step is probably distal to an activation produced by protein-kinase-C-dependent phosphorylation. As is the case for several other K⁺ channels, high concentrations of non-metabolizable ATP analogues inhibit this channel.