Eighty-five patients with essential hypertension received a restricted sodium diet for 10 days. During the last 5 days sodium intake was increased by oral administration of an additional 300 mmol of NaCl. At the end of both periods blood pressure, plasma and urinary concentration of Na and K, and 24-hour secretion rate of aldosterone (ASR) were determined. ASR in the hypertensives was after NaCl loading slightly higher than that in 21 normotensive controls. Aldosterone secretion rates were highest in patients with the lowest plasma K levels after loading with NaCl. This relationship between ASR and potassium was not found after NaCl restriction. After both NaCl restriction and NaCl loading the plasma K concentrations were lowest in patients with the highest mean arterial pressures. From statistical analysis of the relations between plasma Na, plasma K, ASR and mean arterial pressure it seems highly unlikely that aldosterone plays a prime role in the relation between the height of the blood pressure and the concentration of plasma K in essential hypertension. Our data seem in line with the presence of a primary abnormality in electrolyte metabolism in essential hypertension and slight aldosteronism after salt loading as its sequela.