Acute salt loading was performed in three groups of patients, a normal group, a group with essential hypertension, and a group with hypertension due to renal arterial stenosis.Patients with hypertension due to renal arterial stenosis were found to excrete an acute salt load in a qualitatively and quantitatively similar manner to normal individuals, in marked contrast to the exaggerated natriuretic response of individuals with essential hypertension.Of measures in which significant differences were found, the test urine volume, per cent of administered sodium excreted, and the ratios of clearance of sodium in the test and baseline periods yielded a clear separation of patients with essential renal hypertension.The results of this study indicate that acute salt loading in hypertensive patients may have diagnostic and prognostic value.The test procedure is simple, rapid, free from morbidity, and requires a minimum of laboratory analyses.