The effect of calcium supplementation on blood pressure was studied in three groups: eight normotensive subjects, 14 borderline essential hypertensive subjects, and 11 established essential hypertensive subjects (EEH). All subjects were outpatients and were given 6 g of calcium lactate (779 mg of elemental calcium) daily for 16 weeks. In EEH, systolic blood pressure was decreased significantly by 6 mmHg, but diastolic blood pressure was not decreased significantly (2 mmHg). In other groups, blood pressure was not reduced significantly. In all groups, urinary excretion of calcium was increased significantly but urinary excretion of other electrolytes, such as sodium, potassium and phosphate did not change. Serum electrolytes, various hormones, such as parathyroid hormone, 1-25(OH)2D3, 25OHD3, plasma renin activity, plasma aldosterone, plasma norepinephrine and epinephrine, and cardiac output did not change throughout this study. There were no adverse effects observed during this trial. From these results, it was concluded that daily administration of 6 g of calcium lactate produces a slight antihypertensive effect in EEH, and that this might be useful as a supplementary treatment for essential hypertension, especially in the aged.