Compelling evidence shows that magnesium (Mg) content is directly correlated to proliferation in normal cells as Mg stimulates DNA and protein synthesis. Some data have demonstrated that upon mitogenic stimuli normal cell are able to increase their intracellular Mg content, likely by activating Mg influx. Mg deprivation, in turn, induces inhibition of DNA and protein synthesis thus promoting growth arrest. From a mechanistic viewpoint, Mg deprivation may influence cell cycle control by upregulating the cyclin inhibitor p27^Kip1 thus influencing cyclin E-dependent kinases. In many neoplastic cells, Mg is higher than in normal counterparts and this high Mg is maintained also against concentration gradient. Moderate vs. severe and acute vs chronic effect of Mg deprivation must be distinguished: severe Mg deprivation causes growth arrest also in tumor cells, while chronic Mg deprivation leads to an "adaptation" of tumor cells both to growth rate and Mg content. In tumor cells deranged Mg content and distribution is likely due to an inhibition of Mg efflux via the Na-Mg antiport. When differentiation process is induced by receptor mediated stimuli such as IFN-alpha and ATP, decrease of cell Mg content accompanies with activation of Mg efflux. Transformed cells may thus display high growth rate also because they retain a large amount of Mg. On their whole, these data strongly suggest that regulation of intracellular Mg availability parallels the molecular control of cell proliferation, and maybe also cell differentiation and death.