Abstract.Serum and erythrocyte magnesium concentrations (S‐Mg, E‐Mg) were measured in 122 junior high school students followed up for two years from 12 to 14 years of age, and the relationship to blood pressure and a family history of hypertension were investigated. The subjects who had high S‐Mg and E‐Mg levels at the first examination two years prior tended to show high levels after this follow‐up. There were significant positive correlations between two intraindividual values of S‐Mg and E‐Mg. A similar tendency was found for blood pressure. Tracking phenomena were observed with these measures. The subjects who had high E‐Mg levels at the first examination showed no blood pressure elevation during the two‐ year period. The subjects with a family history of hypertension [FH(+)] showed a higher degree of blood pressure rise during two years than those with no family history [FH(‐)], with a significant difference in systolic blood pressure at the age of 14. E‐Mg tended to be lower in the FH(+) group than in the FH(‐) group with a significant difference in 14‐year‐old girls. These results suggest that a hereditary predisposition to hypertension is related to magnesium metabolism and that intracellular magnesium deficiency may influence blood pressure elev