BACKGROUND The aim of the study was to characterize the epithelial transport properties of the distal duodenal mucosa in untreated coeliac disease. METHODS The study was performed in 20 patients and in 22 healthy controls. Net fluid and bicarbonate transport was measured with a triple-lumen perfusion technique. Interdigestive motility was recorded by manometry, and the transmural potential difference (PD) was measured as an indicator of electrogenic anion secretion. RESULTS In the patients a net fluid secretion was seen (-1.04 +/- 0.26 versus 0.12 +/- 0.11 ml/min x 10 cm in the controls; p < 0.01), and there was no significant net bicarbonate absorption (5.6 +/- 3.4 versus 27.5 +/- 4.4 mumol/min x 10 cm in the controls; p < 0.001). The epithelial transport rate varied with the interdigestive motility; a shift in the secretory direction was seen in late phase II of the migrating motor complex (MMC). Moreover, the MMC-related PD curve was significantly displaced in the lumen-negative direction (p < 0.001 versus controls). CONCLUSIONS The results suggest that in untreated coeliac disease, active chloride secretion is enhanced, and Na+/H+ exchange is reduced. This remodelled mucosa still seems to respond to neurogenic stimuli, as suggested by the presence of MMC-related changes in secretion rate.