In the present study, we have investigated the possible consequences of the chloride channel defect in the intestine of cystic fibrosis (CF) patients for electrolyte and water transport in the jejunum in vivo, using a multilumen, double occluding balloon catheter, and an Ag/AgCl intraluminal electrode. During a chloride-free perfusion, to optimize the sensitivity of our measurements, the transmural potential difference (PD)(lumen with reference to serosal side) was found to be significantly higher in the jejunum of CF patients (+ 8.0±2.1 mV; n= 5) than in healthy control subjects (-2.2±2.0 mV; n= 9). The chloride concentration measured in chloride-free jejunal perfusates of CF patients was significantly lower than in controls (10.9±2.3 and 41.4±8.2 mM, respectively). Possible differences in net chloride and water secretion did not reach statistical significance (chloride secretion controls:-2.1±0.9 mmol/10 cm/h; CF:-0.8±0.2 mmol/10 cm/h; water secretion controls:-0.8±2.5 mL/10 cm/h; CF:-11.7±8.9 mL/10 cm/h). In control subjects, intraluminally applied theophylline stimulated the secretion of water (Δ23. 4±4.6 mL/10 cm/h) and chloride (Δ4. 1±1.1 mmol/10 cm/h), but not in CF patients (respectively Δ3. 6±3.3 mL/10 cm/h and Δ1. 1±1.1 mmol/10 cm/h). In controls, theophylline caused a significant increase in lumen negativity (PD-10.2±2.6 mV), but no change could be seen in CF patient transmural PD. These observations provide in vivo evidence for a decreased chloride permeability in the jejunum in CF, resulting in a significant reduction in net electrolyte and water secretion in the presence, but not in the absence, of an intestinal secretagogue.