To determine the influence of dietary sodium intake on the effects of hydrochlorothiazide (HCT) on blood pressure (BP), serum electrolytes, renin and aldosterone, nine male patients with uncomplicated essential hypertension were studied during the following therapeutic regimes: 1) sodium restriction alone (50 mmol/day), 2) sodium restriction combined with HCT (50 MG TWICE DAILY), 3) HCT alone, and 4) sodium restriction combined with HCT. Low sodium diet alone and HCT alone lowered BP to the same extent. The combination of HCT and sodium restriction had no extra effect on supine BP, but elicited complaints of dizziness and weakness in each patient, and overt orthostatic hypotension in three cases. Sodium restriction during HCT treatment caused hyponatraemia and aggravated hypokalaemia. Hyponatraemia could not be accounted for solely by changes in cumulative sodium balance. Plasma renin concentration rose markedly during the combined treatment. Plasma aldosterone was normal during HCT alone, but elevated when HCT was combined with sodium restriction. These results cast some doubt on the therapeutic value of prescribing a low sodium diet to patients with essential hypertension treated with thiazide diuretics. Overactivity of the renin-angiotensin-aldosterone system during this regime might explain both the lack of a beneficial effect on BP and the adverse influence on serum potassium.