Magnesium chloride (MgCl2.6H2O) was administered at dietary levels of 0 (control), 0.3, 0.6, 1.25, 2.5 or 5% to groups of 10 male and 10 female B6C3F1 mice for 13 weeks. In both sexes of the 5% treatment group a decrease in body weight was observed. While clinical signs and hematological or blood biochemistry parameters showed no treatment-related effects, histopathologically, vacuolation of kidney tubular cells was apparent in males of the 2.5 and 5% concentration groups. Thus, the study demonstrated that diet containing over 2.5% MgCl2.6H2O exerts toxic effects in B6C3F1 mice. We therefore conclude that a 2.5% level of MgCl2.6H2O in the diet is the minimal toxic dose.