1. Erythrocyte mass, plasma volume (PV), extracellular fluid volume (ECFV) and total body water were simultaneously measured in 30 normotensive and 30 normal-renin hypertensive Caucasian male subjects for accurate determination of the presence or absence of a disorder(s) in body-fluid composition in hypertension. 2. The results indicate that plasma volume and total blood volume are lower in hypertensive subjects than in normotensive control subjects. The PV comprised 19% of the ECFV in both control and hypertensive subjects. 3. ECFV was lower in hypertensive subjects than in normotensive control subjects; the PV and interstitial fluid components of the ECFV were reduced by similar proportions. The ECFV, furthermore, comprised a smaller portion of the total body water in hypertensive subjects than that in control subjects. 4. We conclude that in the hypertensive state there is a reduction in the ECFV, but that there is no change in the partition of the ECFV between the plasma and interstitial components.