The mammalian collecting duct plays a dominant role in regulating K+excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K+, whereas under K+depletion, the intercalated cell reabsorbs K+. Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K+channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K+secretion and reabsorption. This review summarizes K+transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K+transport is regulated in the collecting duct.