Magnesium is mainly an intracellular cation, and its availability is not represented reliably by plasma levels. The effects of cisplatin on Mg concentrations in plasma (PMg) and erythrocytes (EMg) were investigated in 22 neoplastic patients. Assays were done before and 1, 2, 4 and 7 days after cisplatin administration; 10 patients were also checked following six courses of chemotherapy. PMg decreased progressively either day by day after a single dose of cisplatin (p