Due to the widespread use of magnesium (Mg) in human medicine, interest in quantitative enteral absorption rates arose more than 100 years ago. The introduction of the AAS technique and of radioisotopes has stimulated research. Slowly exchanging compartments have not, however, been accessible up until now, and the exact mechanism(s) of enteral Mg absorption is still unclear; hence, the fate of orally administered Mg cannot be followed in detail. The capacity of skeletal Mg stores varies with exogenous Mg supply, consumption, excretion and age. In otherwise healthy subjects, urine Mg levels correlate with exogenous supply. Neglecting the fact that exogenous Mg exchanges with deep Mg stores to a considerable degree, it makes sense to qualitatively study the availability of Mg compounds by relating the amount administered to the amount excreted in the urine, especially after filling deep compartments. Since renal Mg excretion patterns are known to change when tubular reabsorption capacity is exceeded or when pH alterations occur, these parameters must be monitored and taken into account at the quantitative evaluation of data.