Extensive experimental and clinical studies in animals and humans during the past 25 years have yielded considerable data regarding the availability of internal magnesium (Mg) stores for the maintenance of extracellular and critical tissue concentrations of Mg during periods of Mg deprivation. The bulk of body Mg is present in bone and skeletal muscle, and it is these two sites that provide essentially all the available Mg. In the rat, approximately 15% of bone Mg (equivalent to 1.5 mmol/kg body weight) can be lost, whereas less than one tenth of that amount is available from skeletal muscle. In the human, up to 35% of bone Mg can be lost, but the mean loss in extant studies is 18% (equivalent to 1.2 mmol/kg body weight). In contrast to the rat, up to 40% of human skeletal-muscle Mg can be lost with an average loss of 15% (equivalent to 0.45 mmol/kg). In the human, the bone and skeletal-muscle Mg pools can provide an average of 1.7 mmol/kg body weight equivalent to 15% of total body Mg. Release of Mg from these stores appears to depend on the presence of hypomagnesemia which may also result in small, significant and potentially adverse Mg losses from certain vital organs such as the heart, kidney and brain. The liver and other organs appear not to lose Mg despite Mg deprivation although intracellular Mg shifts of importance cannot be ruled out. These data indicate that the body reserve to combat Mg depletion is not designed to protect the extracellular Mg pool and certain critical organs from Mg deficiency. A continuous optimal intake of Mg is needed for good nutrition and health.