1. The effect of extracellular volume expansion (ECVE) during water diuresis, and of water diuresis alone, on the formation of free water in man was compared.
2. ECVE reduced free water formation at any given rate of distal delivery compared with water diuresis. Thus, ECVE depresses distal sodium chloride reabsorption.
3. This attenuation of free water formation occurred both when urine flow (V/100 ml glomerular filtration rate) and distal chloride delivery [(Cwater + CCl)/100 ml glomerular filtration rate] were used as the terms for distal delivery.
4. We suggest that the distal depression of sodium chloride reabsorption after ECVE is probably due to a direct inhibition of distal sodium chloride transport mechanisms, and not to the flooding of the diluting site by the poorly reabsorbable bicarbonate ion.