Calcium gluconate (22.4 g/day; 2 g/day as Ca) was administered orally for 8 weeks to eight hospitalized elderly patients with essential hypertension in order to confirm the hypotensive effect of oral calcium supplementation and to clarify its hypotensive mechanism by analyzing changes in hormonal factors. After 2 weeks of calcium supplementation, both systolic and diastolic pressures decreased significantly and remained decreased for the duration of the study. An elevation of plasma PGE2 correlated with blood pressure reduction was observed at 2 weeks. Plasma norepinephrine decreased significantly from 4 weeks. Plasma parathyroid hormone decreased significantly from 4 weeks, and 1,25-dihydroxyvitamin D (1,25-[OH]2D) decreased significantly at 8 weeks. The reduction of plasma 1,25-(OH)2D correlated with blood pressure decrease at 8 weeks. The present study indicates that the mechanism of its hypotensive effect is multifactorial and may be different during different phases of calcium supplementation. The suppression of plasma 1,25-(OH)2D following reduction of parathyroid hormone may be involved in the hypotensive effect in the chronic phase of calcium supplementation. Enhancement of PGE2 production in the early phase and suppression of sympathetic nervous activity in the chronic phase may also be factors in blood pressure reduction.