Of all known vertebrate tissues, the kidneys of fish are the champions of Mg transport. They can switch from Mg conservation in fresh water to Mg wasting in seawater. High rates of tubular transport and the ability to alternate between Mg reabsorption and secretion make fish kidneys the model of choice investigating the mechanisms of transepithelial and membrane Mg transport and its regulation by extracellular hormones and intracellular messengers. Studies in isolated proximal tubules indicate active transepithelial Mg transport that requires metabolic energy for both tubular reabsorption and secretion. Whether active transport is primary and mediated by a Mg-pump, or secondary and mediated via cotransport or antiport, is unknown. In fresh water fish, the active transport pathway appears to include a Mg-channel located in brush-border membranes of proximal tubules. Although plasma Mg concentrations are well protected, a primary hormone controlling Mg balance has yet to be identified in any animal. Moreover, the mechanisms of intracellular Mg homeostasis, especially in epithelial cells with high Mg throughput, are unknown. New methods, including Mg imaging and genetic/molecular approaches promise to unravel Mg transport mechanisms in teleost renal tubules.