K § and C1-channel currents in the plasma membrane of isolated canine pancreatic acinar cells were studied by patch-clamp single-channel and whole-cell current recording techniques. In excised inside-out patches, we found a Ca 2+-activated (control range 0.01-0.4 ~tM) and voltage-activated K+-selective channel with a unit conductance of approximately 40 pS in symmetrical K+-rich solutions. In intact cells, addition of acetylcholine (1 ~tM) or bombesin tetradecapeptide (0.1 nM) to the bath evoked an increase in frequency of K § channel opening. In whole-cell recordings on cells dialyzed with K+-rich and Ca2+-free solution containing 0.5 mM EGTA, the resting potential was about -40 mV. Depolarizing voltage pulses activated outward K + currents, which were blocked by 10 mM tetraethylammonium, whereas hyperpolarizing pulses evoked smaller inward currents. Acetylcholine or bombesin activated the K + current and enhanced the inward current, which was reduced by a low CI-(10 mM) intracellular solution at -90 mV holding potential. These results suggest that both Ca 2+-and voltage-activated K + channels and Ca2+-activated C1-channels exist in the plasma membrane of canine pancreatic acinar cells.