In cystic fibrosis the potential difference between interstitial fluid and the surface of respiratory and sweat duct epithelia is abnormally large. This study attempted to confirm this finding and investigate its value for a diagnostic test. With a subcutaneous reference electrode the mean potential difference of the inferior turbinate was -25 mv in 19 patients with cystic fibrosis, which was significantly greater than the difference of -13 mv in 22 normal subjects (p less than 0.02). Of the observations in the patients with cystic fibrosis, 67% were greater than the largest potential difference observed in normal individuals. Potential differences from the lip and forearm were also significantly greater in cystic fibrosis than in normal subjects, but there was considerable overlap. Changes in the electrical properties of nasal epithelium and skin have been confirmed in cystic fibrosis, but these have limited use for diagnosis.