The human body requires mineral salts in order to function, which are also indispensable for proper ionic or hydroelectrolyte balance in order for bodily phenomena to take place normally. This balance is only possible if there is an exact compensation between the intake and output of the different ions; if the contributions through the digestive tract vary normally, the renal control of output is so perfect that some have come to consider the kidney the "body's electrolyte brain" (ORTIZ Y MENDEZ, 11). Among the body's principal ionic components are cations - sodium, potassium, calcium and magnesium - as well as anions - chloride, sulphate and bicarbonate. They must all be kept balanced to reach neutrality, which is only achieved when the respective quantities of anions and cations, expressed in equivalent weights, are equal (MAXWELL AND KLEEMAN, 10). Organic normality requires not only mineral factors, but an adequate balance between them. Thus, with relation to magnesium, the direct actions are of great interest, although those interrelated with other ions, in particular calcium, sodium and potassium, are just as interesting. Indeed, magnesium plays an important role in the formation and function of multiple enzymes, attaching strongly onto the enzyme molecule to form real metalloproteins (VALLE, 16) or integrating into the metal-enzyme or metal-substrate complex. Particularly noteworthy is magnesium's activating action on the ATPase of the cellular membrane and on the enzymes which intervene in the oxidative phosphorilation which is so important at the cellular level. Along with calcium it conditions the digestive enzymatic processes (VAN GEERTRUYDEN, 17), although this does not mean that both ions are exchangeable. Magnesium is actively involved in protein synthesis and in that of nucleic acids, fats, etc. as well as in transmission of nervous impulses, cholinergic activity in the central nervous system, neuromuscular excitability, cardiac dynamics, etc. although such effects are largely conditioned by interactions with other ions. Thus, for neuromuscular excitability sodium and potassium are agonists and calcium and magnesium antagonists, while for cardiac activity sodium and calcium are agonists and potassium and magnesium antagonists (PEACH, 13; SERRANO, 14; ELKINTON, 7; DALLEMAGNE, 3 etc.). In a normal adult, magnesium content is approximately 2,000 mEq (24 g.), 50% of which is found in the mineral portion of the bones, 45% in the intracellular medium and the rest in extracellular fluids. The body's magnesium comes from food, where obtaining the required daily minimum is facilitated by the ample distribution of magnesium in the earth's cortex, in which it is the seventh most common element, being the fourth on the planet and the fifth in the Universe (DICKERSON, 6; WACKERY AND PARISI, 18). Due to their chlorophyll content, plants are frequently considered the main source of dietary magnesium, but the richest foods in magnesium are nuts such as almonds, walnuts, peanuts, hazelnuts, cacao, whole cereals, etc. Among the food items of an animal origin richest in magnesium are cheese, molluscs, crustaceans, fatty fish, etc., with relatively low contents found in meat, white fish, milk and eggs (LAJUSTICIA, 9; PAUL AND SOUTHGATE, 12, etc.). As far as water is concerned, the magnesium content depends on the mineralization of the land where it flows or settles and thus in sedimentary rock the main source is dolomite; in igneous rock, olivine, biotite, augite, etc.; in metamorphic rock, diopside, serpentine, etc. In all cases the quantity of magnesium in the water depends on the solubility of the respective salts and, thus, that of magnesium carbonate depends, at least in part, on the free carbonic it contains and can reach up to 190 ppm of magnesium ion in equilibrium with carbonates; magnesium sulphates and chlorides are very soluble and may contribute up to several thousand parts per million in equilibrium with the chloride and sulphate ions; in the case of silicates the attack of natural agents gradually disintegrates them, facilitating the release of their soluble contents into the water. In general, the magnesium content of surface water varies between 1 an 40 ppm and may exceed 100 ppm in waters which flow through soil rich in magnesium salts (DAVIS AND DE WIEST, 4). Marine waters contain high quantities of magnesium, approximately 4% of total mineralization, placing them in third place quantitatively as far as mineralizing factors. According to JORDAN (8), when the saline content of marine waters is 35 per cubic decimetre, the magnesium concentration exceeds 1.200 ppm. For drinking water, the R.D. 1138/1990 of 14th September, adapted to agree with Community Directive 80/778/EEC, establishes a level of 30 mg/l. as a guideline. As regards bottled waters, regulated by R.D. 1164/1991 of 22nd July, adapted to Community Directive 80/777/1991 of 15th July 1980, referring to the technical health regulations for the preparation, circulation and trading of bottled drinking water, mention may be made of magnesium for natural mineral waters if the magnesium content is greater than 50 mg/l, and of calcium if it contains more than 150 mg/l of calcium. No minimum or maximum values of magnesium or calcium content are established for mineral-medicinal water, although waters in which the contents of these two components exceed 20% of the cationic total expressed in millivales are considered magnesic or calcic waters. In a study we conducted several years ago on the magnesium and calcium contents and the calcium/magnesium ratio in twenty-five bottled mineral waters, the values found were below the guideline levels established by current law for drinking water for public consumption and below the recommended limits for this type of water. Only one water exceeded these values. According to the tests performed at Laboratorio Dr. Oliver-Rodés (Tables I and II) on Fontdor natural mineral water, the values found for magnesium (2.4 mg/l) and calcium (23.4 mg/l) are within the guideline levels established for drinking water for public consumption.