We wished to compare the natriuretic and diuretic effect of extracellular volume expansion (ECVE) with 0.9% NaCl to that obtained with up-to-the-neck immersion (NI) in healthy volunteers. Clearance techniques during maximal antidiuretic hormone suppression were utilized to pinpoint the sites of tubular NaCl reabsorption. Mean glomerular filtration rate (GFR) did not differ significantly between ECVE and NI; mean rate of sodium excretion during ECVE was significantly greater than during NI, whereas mean water clearance for a given rate of distal delivery was higher with NI than with ECVE. Our findings suggest that ECVE is associated with greater natriuresis than NI, despite the equality of filtered sodium loads and of distal deliveries under both these procedures.