Aldosterone is a type of steroid hormone that acts primarily in renal collecting ducts to stimulate reabsorption of Na+as well as secretion of K+and H+. It binds with intracellular receptors in the nucleus that stimulate the expression of several genes. Transcription and subsequent translation result in the production of new proteins that modulate the activity of ionic transport systems located in the apical and basolateral membranes of the target epithelial cells. This review focuses on the cellular mechanisms of aldosterone action on Na+, K+, and H+transport in mammalian renal collecting ducts. Although the cellular actions of aldosterone conform to the general mode of steroid hormone action, numerous questions relating to each step in the process remain unanswered.