To test the hypothesis that NaCl and NaHCO3 have divergent effects on blood pressure, we carried out a randomly allocated, placebo-controlled, crossover trial in 10 mildly hypertensive and 10 normal subjects. They ingested a fixed daily basal diet of 60 mmol sodium and chloride, 60 mmol potassium and 14 mmol calcium. After balance was achieved (4 days), the subjects were randomly assigned to drink 3 liters/day of a NaHCO3-containing mineral water (26.2 mmol/l sodium and 33.03 mmol/l HCO3) or a control solution containing equimolar amounts of cations as the chloride salt for 7 days (total daily sodium 138 mmol). All urine was collected. Blood pressure was determined by an automated device. One month later the opposite regimen was followed. NaCl did not influence blood pressure, whereas NaHCO3 decreased systolic blood pressure (by 5 mmHg) in the hypertensive subjects. Both regimens decreased plasma renin activity in the hypertensive subjects but did not consistently influence plasma aldosterone or catecholamines. However, urinary calcium excretion, which was greater in hypertensives than in normotensives, and greater in white than in black subjects, increased consistently with NaCl but not with NaHCO3. The excretion of urate was not influenced by the regimens; however, urate excretion was consistently greater in whites than in blacks. The data show that NaCl increases calcium excretion whereas NaHCO3 does not, even at modest levels of intake. NaCl and NaHCO3 may therefore differ in their effects on blood pressure.