We performed a randomised, placebo-controlled, crossover study to examine the effects of sodium and potassium supplementation on blood pressure (BP) and arterial stiffness in untreated (pre)hypertensive individuals. During the study, subjects were on a fully controlled diet that was relatively low in sodium and potassium. After a 1-week run-in period, subjects received capsules with supplemental sodium (3¿g¿d-1, equals 7.6¿g¿d-1 of salt), supplemental potassium (3¿g¿d-1) or placebo, for 4 weeks each, in random order. Fasting office BP, 24-h ambulatory BP and measures of arterial stiffness were assessed at baseline and every 4 weeks. Of 37 randomized subjects, 36 completed the study. They had a mean pre-treatment BP of 145/81¿mm¿Hg and 69% had systolic BP greater than or equal to140¿mm¿Hg. Sodium excretion was increased by 98 mmol per 24¿h and potassium excretion by 63 mmol per 24¿h during active interventions, compared with placebo. During sodium supplementation, office BP was significantly increased by 7.5/3.3¿mm¿Hg, 24-h BP by 7.5/2.7¿mm¿Hg and central BP by 8.5/3.6¿mm¿Hg. During potassium supplementation, 24-h BP was significantly reduced by 3.9/1.6¿mm¿Hg and central pulse pressure by 2.9¿mm¿Hg. Pulse wave velocity and augmentation index were not significantly affected by sodium or potassium supplementation. In conclusion, increasing the intake of sodium caused a substantial increase in BP in subjects with untreated elevated BP. Increased potassium intake, on top of a relatively low-sodium diet, had a beneficial effect on BP. Arterial stiffness did not materially change during 4-week interventions with sodium or potassium.