Thirty men with essential hypertension were examined at three different levels of sodium intake, containing 135, 44 and 290 mmol sodium per day, respectively. Ten patients who increased their 24 hour mean ambulatory blood pressure 10% or more when going from low to high sodium intake were defined as salt sensitive, the others as salt resistant. The casual and 24 hour ambulatory blood pressure measurements defined partly different patients as salt sensitive. In multiple regression analysis, salt sensitivity was associated with an increase in diuresis during low sodium intake, demonstrating a dissociation between water and sodium excretion during salt depletion in the salt sensitive group. The change 24 hour ambulatory blood pressure during salt repletion was positively correlated to the increase in the atrial natriuretic peptide (p < 0.01), and inversely correlated to the plasma concentration of atrial natriuretic peptide after salt depletion (p < 0.01). No difference in plasma norepinephrine, renin, aldosterone, plasma volume, blood volume or 24 hour sodium excretion was found between salt sensitive and salt resistant subjects. We conclude that salt sensitivity is difficult to describe as an entity, but seems to be associated with lower levels of atrial natriuretic peptide and a different response to salt depletion.