Background: Oesophageal intraluminal impedance is currently used for assessment of reflux in gastrooesophageal reflux disease (GORD). Oesophageal mucosa integrity may have a key role in heartburn perception in non-erosive reflux disease (NERD). Severe erosive oesophagitis is associated with low impedance baseline. We hypothesised that impedance baseline measurements could be used to evaluate changes in oesophageal mucosa integrity in man. Methods: We measured oesophageal impedance baseline before, during and after acid perfusion in rabbits and healthy subjects. Transepithelial resistance (TER) was determined and dilated intercellular spaces (DIS) were assessed in isolated rabbit oesophageal mucosa. Impedance baseline was measured retrospectively at different levels of the oesophagus in impedance-pH recordings from asymptomatic volunteers and patients with GORD. Results: In healthy subjects and rabbits, impedance baseline dropped dramatically during perfusion of control solution (pH 7.2) but after perfusion, impedance recovered. In rabbits, after perfusion with saline pH 1.5 and 1.0 impedance values remained a 39.1±7.0% and 63.9±6.5% (p<0.05) lower respectively. There was a positive correlation between in vivo basal impedance and in vitro TER values (r=0.72, p=0.0021). Tissue showed no erosions but both acidic solutions induced DIS. In healthy subjects, after perfusion with saline pH 2.0 and 1.0 the impedance baseline remained lower a 21.9±6.5% and 52.7±5.0%, (p<0.0001) respectively. Patients with GORD have a lower impedance baseline than healthy volunteers at the distal oesophagus. Conclusions: Impedance baseline measurements might be used to evaluate the status of the oesophageal mucosa and to study the role of the impaired mucosal integrity in acid-induced heartburn in healthy volunteers and in patients with GORD.