In populations, dietary intakes of potassium, calcium, and magnesium each have been inversely associated with blood pressure. However, most clinical trials in normotensive populations have not found that dietary supplements of these minerals lowered blood pressure. We tested the hypothesis that normotensive persons who have low habitual intake of these minerals would be particularly responsive to supplementation. Three hundred normotensive women in the Nurses Health Study II (mean age, 39 years), whose reported intakes of potassium, calcium, and magnesium were between the 10th and 15th percentiles, received for 16 weeks' duration daily supplements of either potassium 40 mmol, calcium 30 mmol (1200 mg), magnesium 14 mmol (336 mg), all three minerals together or placebos. At baseline, mean (+/-SD) 24-hour ambulatory blood pressures were 116+/-8 and 73+/-6 mm Hg systolic and diastolic, respectively, and mean dietary intakes of potassium, calcium, and magnesium were 62+/-20 mmol/d, 638+/-265 mg/d, and 239+/-79 mg/d, respectively. The mean differences (with 95% confidence intervals) of the changes in systolic and diastolic blood pressures between the treatment and placebo groups were significant for potassium, -2.0 (-3.7 to -0.3) and -1.7 (-3.0 to -0.4), but not for calcium, -0.6 (-2.2 to 1.0) and -0.7 (-2.0 to 0.6), or for magnesium, -0.9 (-2.6 to 0.8) and -0.7 (-2.2 to 0.8). The administration of calcium and magnesium with potassium did not enhance the effect of potassium alone, and the changes in blood pressure were not significant -1.3 (-3.0 to 0.4) and -0.9 (-2.2 to 0.4). In conclusion, potassium, but not calcium or magnesium supplements, has a modest blood pressure-lowering effect in normotensive persons with low dietary intake. This study strengthens evidence for the importance of potassium for blood pressure regulation in the general population.