In this study, the effects of oral magnesium supplementation were evaluated on plasma, erythrocyte, and urinary cation levels. The study was carried out with an open, cross-over and randomized design. Healthy subjects received two sachets per day of magnesium (366 mg Mg2+/d) for two 28 d cycles, separated by a 3-week washout interval. Magnesium concentrations were measured before the supplementation and at weekly intervals during the treatment. Urinary excretion of Mg was significantly increased during supplementation, with no differences among the weeks examined. Only small increases in plasma magnesium occurred, and values returned to the prestudy levels before the end of supplementation. The increase in erythrocyte magnesium, however, was statistically significant; this trend suggests the existence of a saturable mechanisms, which prevents any possible magnesium overload during oral supplementation. We conclude that erythrocytes might be considered a suitable index for evaluating the bioavailability of magnesium salts in marginally magnesium-deficient subjects.