1. Free water clearance (CH2O) was measured in sixteen normal subjects and twenty-five patients with uncomplicated hypertension at different levels of salt intake.
2. In normal subjects CH2O and Na+ excretion were related thus: log Na+ excretion = 0·1685 + 0·1508 × CH2O. At a given value for Na+ excretion the standard deviation for CH2O was ±1·5 ml min−1.
3. The mean value for the ratio CH2O/creatinine clearance was higher in the hypertensive patients than in controls at all levels of Na+ excretion.
4. Most previous studies reporting decreased values for CH2O in hypertension can be explained by the exponential relationship between CH2O and Na+ excretion.