Salt sensitivity has been implicated in the age-related increase in blood pressure. We studied the reproducibility of a rapid method for assessing sodium sensitivity and resistance of blood pressure as well as the effect of age on this phenomenon. Blood pressure after volume expansion with 2 l intravenous saline (0.9%) over 4 hours was compared with that after 1 day of 10 mmol sodium chloride intake and 3 and 40 mg oral doses of furosemide. Normal and hypertensive subjects (n = 28) were studied twice within a year. Cross-sectional observations of the effect of age were made from studies in 230 hypertensive and 430 normotensive subjects. Longitudinal observations of blood pressure change over time were made 10 or more years after categorization of sodium responsivity in 31 subjects. The blood pressure response was reproducible in 28 subjects studied twice (r = 0.56, p less than 0.002). Four subjects changed salt-responsiveness status and six were indeterminate on restudy. Sodium sensitivity of blood pressure increased significantly with increasing age in the entire population (n = 660, r = -0.38, p less than 0.001). The relation was more striking in hypertensive subjects (n = 230, r = -0.31, p less than 0.001) in whom a progressive increase in salt sensitivity with decades was seen than in the normotensive group (n = 430, r = -0.19, p less than 0.01) in whom salt sensitivity was not observed until the sixth decade. Salt-sensitive subjects had a significantly greater increase in systolic (p less than 0.001) and diastolic (p less than 0.01) pressure over time than those who were salt-resistant. Salt sensitivity is a reproducible phenomenon that is related to the age-associated increase in blood pressure characteristic of industrialized societies. In addition, salt sensitivity can be shown to be a predictor of subsequent, age-related blood pressure increase.