The present study has investigated whether an increased natriuresis could account for the hypotensive effect of a high calcium diet which has been reported by others. A calcium supplement (equivalent to 1 g of elemental calcium) was given for 5 days to 18 patients with essential hypertension in a randomized single-blind, placebo-controlled, cross-over trial. In 15 of the patients, 2 liters of isotonic saline were infused intravenously over 4 h during the last day of each test period and hourly urine collections were taken. Calcium supplementation produced a mild but significant hypercalcemia as well as increased urinary calcium excretion. Body weight and systolic blood pressure decreased significantly. The blood pressure decrease was indirectly related to the pretreatment plasma renin activity (r = -0.61, p < 0.01). Urinary sodium excretion increased during calcium diet (80 mmol/day negative balance, p < 0.01). During saline infusion under calcium supplementation the urine volume, osmolality and sodium excretion were significantly higher compared with placebo. The changes in urinary sodium excretion correlated positively with the changes in urinary calcium excretion (r = 0.68, p < 0.01) in patients given the high calcium diet, when infused with saline. We conclude that calcium supplementation induces a considerable sodium loss in the urine which is very likely to result in the hypotensive effect.