Tubular handling of sodium in hypertensive patients was evaluated with urinary phosphate excretion used as a marker for proximal tubular reabsorptive capacity. 9 hypertensive patients and 9 normal control subjects were studied during sustained water diuresis and the i.v. infusion of isotonic sodium chloride solution to produce volume expansion. In the hypertensive patients there was exaggerated phosphaturia, natriuresis and enhanced distal delivery of sodium. Sodium reabsorption in the diluting segment was normal. The enhanced distal delivery and augmented phosphaturia suggest that a decreased reabsorption of sodium in the proximal tubule is the most likely explanation for the exaggerated natriuretic response to volume expansion in hypertensive patients.