In a comparative study the influence of changes in dietary sodium intake on blood pressure, renal function, extracellular fluid volume, the renin-angiotensin-aldosterone system and plasma concentrations of arginine vasopressin, atrial natriuretic factor and cyclic guanosine monophosphate (GMP) was investigated in 12 patients with essential hypertension and in 10 normotensive controls. The subjects were studied after 4 days on a low (50 mmol/day), medium (180 mmol/day) or high (380 mmol/day) sodium intake. Renal sodium handling was assessed by simultaneous measurements of 51Cr-ethylenediaminetetraacetic acid (EDTA), lithium and sodium clearances. Identical values for the extracellular fluid volume, glomerular filtration rate and proximal and distal tubular resorption rates of sodium and water were found in the hypertensive patients and the controls at all three levels of sodium intake. In both groups, raising the sodium intake from low to high significantly increased 51Cr-EDTA and lithium clearance (an indirect measure of end-proximal fluid delivery), with intermediate values for the medium-sodium diet. The estimated values of fractional proximal and distal sodium resorption decreased when sodium intake was raised; the absolute proximal sodium resorption rate did not change, whereas the absolute distal sodium resorption rate as well as the extracellular fluid volume and sodium clearance increased. Blood pressure and the heart rate were unaffected by sodium intake. In both hypertensives and controls, plasma concentrations of active renin, angiotensin II and aldosterone decreased with increasing sodium intake, arginine vasopressin did not change, and atrial natriuretic factor and cyclic GMP increased.(ABSTRACT TRUNCATED AT 250 WORDS)