Twenty patients with mild or moderate essential hypertension and not receiving any drug treatment, who had been moderately restricting their sodium intake to around 70 mmol(mEq) a day for at least one month and whose mean blood pressure was then 163/103 mm Hg, were entered into a double blind, randomised crossover study to compare one month's treatment with slow release potassium chloride tablets (64 mmol potassium chloride a day) with one month's treatment with a matching placebo. Mean (SEM) urinary sodium excretion on entry to the study was 68 (6.8) mmol/24 h. Mean urinary potassium excretion increased from 67 (6.9) mmol(mEq)/24 h with placebo to 117 (4.6) mmol/24 h with potassium chloride. Supine and standing systolic and diastolic blood pressures did not change significantly with potassium chloride supplementation when compared with pressures while receiving placebo or before randomisation. In patients who are able moderately to restrict their sodium intake doubling potassium as a chloride salt has little or no effect on blood pressure.