Various tubular functions were assessed in 10 patients with polycystic kidney disease (PKD). Three relatively unique abnormalities were apparent in many of these patients -- an inability to maximally concentrate urine, a decrease in ability to lower urine pH after acute acid challenge, and an inability to excrete adequate amounts of ammonium during persistent acid challenge. The defects in urinary acidification and ammonium excretion in PKD have not been previously described.