1. Ten patients with essential hypertension and ten normotensive control subjects were investigated before and after an isotonic volume load.
2. The increases in the clearances of lithium and of sodium were greater in the hypertensive than in the normotensive subjects (P < 0.05). The fractional increases showed a positive correlation (rs = 0.68, P < 0.05) in the hypertensive patients.
3. An increase in glomerular filtration rate in the hypertensive patients correlated positively with the increase in proximal output (rs = 0.70, P < 0.05), but did not correlate with the change in sodium excretion.
4. The exaggerated natriuresis was accompanied by a significant rise in the absolute rate of sodium reabsorption in the distal segments, but distal fractional reabsorption decreased.
5. It is concluded that the exaggerated natriuresis can be accounted for by an increase in the proximal output of sodium and water.