Mast cells, multifunctional effector cells of the immune system, are implicated in the pathogenesis of hepatic steatosis and fibrosis. Magnesium (Mg) deficiency was reported to increase triglyceride concentration in the liver, and to exacerbate the collagen deposition induced by carbon tetrachloride in the liver. Although Mg deficiency increases mast cells in the small intestine, the kidney and bone marrow, the effect of Mg deficiency on mast cells has not been clarified in the liver. We examined the emergence of mast cells in the liver of Sprague-Dawley rats given an Mg-deficient diet. Rats were fed a control diet or an Mg-deficient diet for 4 wk. Mg deficiency increased the levels of mRNA known to be expressed by mast cells in the liver; the mRNA of α- and β-chain high-affinity immunoglobulin E receptor (FcεR1α, FcεR1β), and the mRNA of mast cell protease 1 (Mcpt1), and mast cell protease 2 (Mcpt2). Histological observation showed that some mast cells were locally distributed around portal triads in the Mg-deficient group but mast cells were scarcely found in the control group. These results clearly indicate that Mg deficiency induces the emergence of mast cells around portal triads of the liver in Sprague-Dawley rats.