Ion-selective electrodes have recently been designed for determining the ionized concentration of magnesium (Mg2+) in serum. This development may allow new insights into some metabolic diseases of cattle. For this report, the concentrations of Mg2+, total magnesium (Mg-tot), ionized calcium (Ca2+), total calcium (Ca-tot), and inorganic phosphate (P-i) were determined in sera from seventeen 3- to 16-year-old Brown Swiss and crossed Simmental/Red Holstein cows during the periparturient period. In each animal, a transient increase of Mg2+ and Mg-tot serum concentrations was observed in association with the transient decrease in serum concentrations of Ca2+, Ca-tot and P-i after parturition. On average, throughout the study, the serum Mg2+ concentrations were 68.5% of those of Mg-tot, whereas the serum Ca2+ concentrations were 52% of those of Ca-tot. The possible mechanisms involved in the transient increase of Mg2+ and Mg-tot serum concentrations are discussed.