To investigate the antihypertensive effect of moderate sodium restriction, the sodium intake of 11 male outpatients was reduced by 120 mmol/day for 4-6 weeks. These patients and an untreated control group were slightly obese and had mild untreated hypertension (WHO 1-2). All subjects were examined before and at the end of the experiment. Diastolic blood pressure fell significantly in the diet group in comparison with the control group. Invasive haemodynamic examinations in the diet group showed an unchanged mean cardiac output and a reduction of mean total peripheral resistance. Plasma volume (Evan's Blue) did not change, neither did extracellular volume as calculated from determinations of tritiated water, total body potassium and body mass. During sodium restriction, plasma renin activity and urinary aldosterone excretion significantly increased. Noradrenaline and dopamine excretion in urine showed no significant changes during sodium restriction, neither did the plasma concentrations of atrial natriuretic peptides. The reduction in mean arterial blood pressure was correlated significantly with a decrease in 24-h sodium excretion and an increase in urinary aldosterone excretion. In conclusion, moderate dietary sodium restriction seems to lower blood pressure by diminishing the total peripheral resistance while cardiac output, extracellular and intravascular volumes are maintained.