Potassium channels in the apical and basolateral membranes of tubule cells serve several important functions. They contribute to the generation of the cell-negative potential, mediate volume reductions following cell swelling and play a key role in secretion of potassium in both the thick ascending limb of Henle's loop and principal tubule cells of the initial and cortical collecting tubules. Secretion of potassium occurs via a well-defined class of potassium channels distinguished by their low single channel conductance, mild inward rectification, high sensitivity to inhibition by low pH, millimolar concentrations of ATP, arachidonic acid and PKC, and stimulation by vasopressin and pretreatment with a high potassium diet. Genes encoding several isoforms of this channel have been cloned and the proteins located to the apical membranes of cells lining the thick ascending limb of Henle's loop and the collecting tubules, and progress made concerning their structure-function relationship.