CNP in cor pulmonale and heart failure than as a circulating hormone involved in cardiovascular homeostasis. Thus, measurement of plasma CNP levels and studies involving systemic infusions of CNP may have only a limited role in clarifying its physiological significance. Background-Normal subjects have a negative nasal transmucosal potential difference (TPD) at rest which becomes more negative with exercise. Patients with cystic fibrosis have a more negative resting nasal TPD than controls. The present study was designed to determine the effects of exercise on the TPD of patients with cystic fibrosis. Methods -Seven subjects with cystic fibrosis and seven control subjects had their usual TPD measured at rest, and during and after a 12 minute period on an exercise bicycle designed to produce a pulse rate of 80% of their maximum predicted value. Results -The normal subjects developed a more negative nasal TPD during exercise which returned towards normal at the completion ofthe rest period. The patients with cystic fibrosis had higher resting values which became less negative during exercise. At the end of the exercise period there was no difference between the two groups. At the end of the recovery period