Aldosterone is a highly important mineralocorticoid produced in the zona glomerulosa of the adrenal cortex in response to angiotensin II, potassium ions, and adrenocorticotropic hormone. Its most important physiological functions concern maintenance of sodium balance, potassium homeostasis, and excretion of hydrogen ions. Although the distal tubule of the kidney appears to be its principal site of action, its effects on other membranes and glandular tissues may induce secondary effects on other portions of the nephron. Its precise mechanism of action is the result of its reaction with highly specific protein receptors located only in responsive tissues. These proteins, in complex with aldosterone, stimulate formation of DNA-dependent RNA that in turn leads to synthesis of new proteins. The latter, by an unknown mechanism, are responsible for the definitive action of aldosterone.