Studies of the postnatal development of the pancrease in CF infants show a failure of acinar development and an increase in lumen volume with accumulation of secretory material within the pancreatic ducts. Our evaluation of functional changes within the exocrine pancreas are consistent with the pathologic findings. Impaired pancreatic fluid secretion appears to be a primary phenomenon of CF. We have shown that pancreatic secretions from CF patients also contain significantly higher concentrations of protein in comparison with pancreatic function-matched controls. Since total protein output is not increased, hyperconcentration of protein appears to be a direct result of a primary defect of fluid secretion. Protein hyperconcentration appears to predispose CF patients to protein precipitation and obstruction within small pancreatic ducts which, in turn, produces pancreatic acinar atrophy and fibrosis Our studies suggest that both bicarbonate and chloride transport within pancreatic ducts account for deficient fluid secretion in the pancreas of CF subjects.