The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. The transport process mediated by Na-K-Cl cotransporters is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the "loop" diuretics bumetanide, benzmetanide, and furosemide. Presently, two distinct Na-K-Cl cotransporter isoforms have been identified by cDNA cloning and expression; genes encoding these two isoforms are located on different chromosomes and their gene products share approximately 60% amino acid sequence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epithelia with the Na-K-Cl cotransporter localized to the basolateral membrane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, localized to the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion. The two Na-K-Cl cotransporter isoforms are also part of a superfamily of cation-chloride cotransporters, which includes electroneutral K-Cl and Na-Cl cotransporters. Na-K-Cl cotransporter activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particularly of the NKCCI isoform) occurs through direct phosphorylation/dephosphorylation of the cotransport protein itself though the specific protein kinases involved remain unknown. An important regulator of cotransporter activity in secretory epithelia and other cells as well is intracellular [Cl] ([Cl]i), with a reduction in [Cl]i being the apparent means by which basolateral Na-K-Cl cotransport activity is increased and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.