To determine the effects of potassium on blood pressure and factors affecting blood pressure, we conducted a randomized, placebo controlled trial of a potassium chloride-based substitute for table salt in 23 patients with mild to moderate essential hypertension. In addition, the effects of potassium chloride on sodium balance were studied in 10 normal subjects. Potassium loading with 100 mmol/day over five days in these normal subjects caused a cumulative negative sodium balance of 138 +/- 35 mmol, similar in degree to that achieved by severe dietary sodium restriction. However, two weeks of potassium treatment (100 mmol/day) in patients with essential hypertension did not lower blood pressure (BP) either in the supine or upright positions (potassium treatment: mean BP 108 +/- 3 lying and 113 +/- 3 mmHg standing; placebo treatment: mean BP 109 +/- 3 lying and 115 +/- 3 mmHg standing). Patients found it difficult to tolerate the potassium-based salt substitute in the dose given. We conclude that it is premature to recommend an increase in potassium chloride intake as treatment for raised blood pressure.