BACKGROUND AND OBJECTIVES: Previous studies reported that sodium and potassium play an important role in the pathogenesis of hypertension. Recently attention has been directed towards a possible role of the divalent cations such as calcium, and magnesium. Plasma renin activity is also known to be related to divalent cations heterogeneously. This study investigated the relationships between serum magnesium and ionized calcium and plasma renin activity. MATERIALS AND METHODS: The subjects consisted of 27 essential hypertensive patients and 25 normotensive controls. Criteria for hypertensive group in this study were systolic blood pressure> or =140mmHg or a diastolic blood pressure > or =90mmHg (JNC-VI, 1997). Inclusion criteria were normal urinalysis, no history of systemic illness, no intake of antihypertensive drugs, and no recent intake of any other medication. We took magnesium-loading test for a reliable method of assessing possible magnesium deficiency. RESULTS: There was no significant difference between two groups in serum Magnesium concentration and other electrolytes and plasma renin activity. There was significantly higher rate in hypertensives than in normotensives in magnesium retention(hypertensive vs. normotensive: 63.56+/-12.21% vs. 38.43+/-11.53%, P<0.001). There was significant differences in ionized calcium between high-renin and low-or normo-renin hypertensives(P<0.001). Plasma renin activity was correlated positively with serum ionized calcium in hypertensives(r=.8147; P<0.001). CONCLUSION: These results suggest that plasma renin activity is a factor that can influence on serum ionized calcium in high-renin hypertensives.