It has been demonstrated that an exaggerated natriuretic response to central hypervolaemia is not necessarily associated with hypertension; many hypertensive subjects manifest either an appropriate or a blunted natriuresis in response to ECFV expansion attained by head-out water immersion. In this study, we tested the hypothesis that an underlying condition of salt-sensitivity may explain the heterogeneity of the natriuretic response of essential hypertension. Both salt-sensitivity tests and 2h water-immersion studies were randomly performed in 18 untreated essential hypertensives under a selected and controlled diet. Salt-sensitivity was defined as a significant drop in mean arterial pressure of 10% or greater, calculated as the difference between the average of the 25 readings under the high and the low salt period. Water immersion did result in a significant natriuretic and calciuretic response in the whole hypertensive group (n = 18, p < 0.001 and p < 0.05, respectively), while the examination of the individual excretion disclosed either exaggerated and appropriate or blunted urinary response. When the hypertensive group was classified in relation to salt-sensitivity, the greater fall in mean arterial pressure during low salt diet (salt-sensitivity) was associated with the more pronounced natriuretic response during water immersion (r = -0.66, p < 0.003). An identical correlation (r = -0.58, p < 0.01) was also found between changes in mean arterial pressure (low salt diet) and urinary calcium excretion (water immersion) in the same hypertensives. The water immersion-induced suppression of plasma aldosterone and the increase in plasma atrial natriuretic peptide did result from comparable magnitude in the salt-sensitive and in salt-resistant subjects.(ABSTRACT TRUNCATED AT 250 WORDS)