1. Arterial pressure, plasma electrolytes and exchangeable sodium were measured in 91 patients with essential hypertension and in 121 normal control subjects. Total body sodium, exchangeable potassium and total body potassium were also measured in some of the hypertensive patients.
2. Mean plasma sodium concentration was slightly but significantly lower in the hypertensive patients as a group, but mean values for other electrolyte measurements were close to normal or predicted normal.
3. Exchangeable sodium was not related to arterial pressure in normal subjects but in hypertensive patients exchangeable sodium correlated significantly with systolic and diastolic pressures. These correlations were significant with two methods of expressing exchangeable sodium, in the whole group of patients, in men and in older patients. Exchangeable sodium was not significantly related to arterial pressure in young patients.
4. Total body sodium also correlated significantly with systolic and diastolic pressures in hypertensive patients.
5. Exchangeable sodium was significantly related to age in hypertensive patients but not in normal subjects. Mean exchangeable sodium was significantly lower than normal in young patients.
6. Plasma potassium concentration was not related to arterial pressure in normal subjects but in essential hypertensive patients plasma potassium concentration, exchangeable potassium and total body potassium correlated negatively with systolic and diastolic pressures. These correlations were also significant in young, but not in old patients.