In a previous study [26] we described the properties of potassium channels in cultured respiratory cells derived from cystic fibrosis patients (CF) and normal individuals (N). In the present study we examine the regulatory mechanisms of these channels by the patch clamp technique. Since there were no apparent differences in the properties of CF and N K+ channels the results were pooled. In the excised inside/out configuration the channel was blocked by different K+ channel blockers. Barium (5.10(-3) mol/l), tetraethylammoniumchloride (5.10(-3) mol/l), quinidine (10(-3) mol/l) and lidocaine (5.10(-3) mol/l), when added to the cytosolic side, inhibited K+ channels reversibly. An increase in the calcium concentration from 10(-7) mol/l to 10(-6) mol/l led to a marked increase in the open channel probability (Po). Further increases in Ca2+ concentration increased Po only slightly. No pH effects on the cytosolic side of the channel were observed. The channel open probability was reduced when ATP was present on the cytosolic side at a concentration of 10(-4) mol/l to 10(-3) mol/l. Non hydrolysable adenosine 5'-[beta, gamma-methylene] triphosphate had the same inhibitory effect as ATP. The inhibition by ATP was blunted by the simultaneous addition of 1 mmol/l ADP. The inhibition of K+ channels by cytosolic ATP may represent a channel regulatory mechanism in the intact cell. This would allow for coupling between the activity of the (Na+ + K+)-pump and the basolateral K+ conductance.