Magnesium (Mg) is essential for cell functions such as the transport of calcium and potassium ions, and modulates signal transduction, energy metabolism, and cell proliferation. Although mice have been used as models of various neurological diseases of humans, and for investigating the therapeutic effects of Mg, neither the normal concentration of Mg in cerebrospinal fluid (CSF), nor its response to alteration of the serum level of Mg has yet been reported. The present study investigated the normal Mg concentration in the CSF of C57BL/6J (B6) and ICR mice and its response to elevation of the serum Mg level in B6 mice. In B6 mice, the normal Mg concentration in the CSF was 0.89 ± 0.11 mM, being lower than that in serum, which was 1.38 ± 0.12 mM, whereas in ICR mice the corresponding values were 1.00 ± 0.12 mM and 1.10 ± 0.09 mM, respectively. No significant alteration was found in the CSF of B6 mice injected intraperitoneally with Mg, even though the serum Mg concentration was significantly increased.