We studied natriuresis during central hypervolaemia by immersing eight normal subjects and eight patients with uncomplicated essential hypertension up to the neck in water, either in the absence (study 1) or presence (study 2) of dopamine blockade by metoclopramide. Water immersion without metoclopramide induced an exaggerated natriuresis in hypertensives compared with normotensives (P less than 0.001). This occurred in the presence of identical hormonal (plasma renin activity, plasma aldosterone and prolactin), renal (creatinine clearance) and pressor responses in both groups (study 1). The marked natriuresis seen during water immersion alone in normotensives was significantly blunted (P less than 0.02) but not abolished during water immersion with addition of metoclopramide. On the other hand, the exaggerated natriuresis found in hypertensives during water immersion alone was completely abolished during water immersion plus dopamine blockade by metoclopramide (study 2). Similar hormonal, renal and pressor changes were detected in both normotensive and hypertensive subjects during water immersion plus metoclopramide administration. Our data demonstrate that metoclopramide abolishes the exaggerated natriuretic response seen in hypertensives during volume expansion produced by water immersion, and suggest that dopamine may play a critical role in mediating the hypernatriuresis of essential hypertension.