The present study was aimed at evaluating the age-dependent alterations of magnesium levels, and the effect of magnesium supplementation on magnesium and protein levels in various body compartments during the maturation process of rats until the young-adult age. The measurements were performed on untreated male Wistar rats of 45, 90, 135 and 180 days of age. In another experiment, the same type of rats (aged 45 and 90 days), were drinking either tap water (with 15 mg/l magnesium), or magnesium acetate solutions (containing 100, 180 or 330 mg/l magnesium) for a period of 63 days. The results suggested that neither the maturation process itself until the young-adult age, nor magnesium supplementation to young rats affect the total magnesium concentrations and protein levels in the plasma, erythrocytes and femoral bones. Although these negative results are in agreement with some literary data, further experiments would be required to clarify the true age-dependent role of magnesium. It will be necessary to include the measurements of ionized magnesium, and to perform supplementation studies with higher magnesium doses for longer periods, extended also to other tissues of really old rats.