Fine control of renal water and electrolyte excretion takes place in the collecting duct, a tubule segment which is also a major site of K+ secretion and hormone action. With the introduction of patch clamp techniques it has been possible to define the contribution of ion channels to K+ transport. Two types of channels have been identified in the cortical collecting tubules of the rabbit and rat: (1) a maxi- or high conductance K+ channel (single channel conductance greater than 80 pS) found only in the apical membrane, and (2) smaller conductance K+ channels (single channel conductance less than 60 pS) found in both apical and basolateral membranes. The gating properties of the K+ channels with smaller conductances differ in the apical and basolateral cell membranes; whereas the open probability of the small conductance K+ channel in the apical membrane is not voltage-sensitive, that of the basolateral channel increases with hyperpolarization. The maxi-K+ channel, so far only found in the apical cell membrane, is voltage-gated but its open probability increases with cell depolarization. The possible role of these K+ channels in different states of the K+ transport system in collecting ducts is discussed.