Blood pressure responses to 1 week of low (20 mmol sodium/day) and high (300 mmol sodium/day) salt intake were investigated in a double-blind, randomized study in 46 white, nonobese subjects with essential hypertension (13 women, 33 men; mean age 45.3 +/- 2.2 years, age range 25 to 80 years). The individuals were classified as salt-sensitive when mean arterial blood pressure rose by at least 5 mm Hg during high salt intake, as salt-resistant when mean arterial blood pressure changed by less than 5 mm Hg, and as "counter-regulators" when mean arterial blood pressure fell by at least 5 mm Hg during the high salt diet. Mean arterial blood pressure of all subjects taken together increased from 101.9 +/- 1.4 mm Hg during salt restriction to 103.7 +/- 1.5 mm Hg (P < .05) during salt loading. Eleven subjects (23.9%) were classified as salt-sensitive, 27 (58.7%) as salt-resistant, and 8 (17.4%) as counter-regulators. Multiple regression analysis revealed that age, but not baseline blood pressure, sex, body mass index, or family history of hypertension contributed significantly to the change in blood pressure following the diets. Ten of the 11 salt-sensitive subjects were older than the median age of 45 years. In salt-sensitive, as compared to salt-resistant, hypertensive subjects, creatinine clearance was lower and plasma renin activity was suppressed at baseline as well as during low and high salt intake. In contrast, plasma concentrations of norepinephrine and atrial natriuretic peptide were elevated in salt-sensitive subjects. These differences between the groups appeared, at least partially, to be age-related.(ABSTRACT TRUNCATED AT 250 WORDS)