Volume flow (J _v), potential difference (ΔΨ), shortcircuit current (i ₀) and electrical resistance (R) were measured simultaneously across bovine tracheal epithelium in vitro. Under basal conditions, with no applied hydrostatic or osmotic pressure gradient (ΔP=0, ΔΠ=0), no spontaneousJ _v was observed. ΔΨ was 31±2 mV (lumen negative),i ₀ 161±8 μA cm⁻² andR 202±9 Ω cm²,n=50. When a ΔΠ was applied, by adding 20–80 mM sucrose into the medium bathing either the luminal or the serosal side of the tissue, a linear relationship was found between ΔΠ andJ _v toward the lumen or toward the serosa. The apparent hydraulic conductivity (apparentL _p) was 4.6–4.910⁻⁶ cm s⁻¹ atm⁻¹. Histamine 10⁻⁴ M did not induce any spontaneousJ _v under basal conditions and had no effect oni ₀ nor onR. However, histamine caused a 100% increase inJ _v elicited by sucrose gradients. It was concluded that histamine exerts a selective action on the hydraulic conductivity of bovine tracheal epithelium. Experiments using H₁-receptors antagonists (diphenhydramine, dimetindene, chloropyramine) and H₂-antagonists (cimetidine, metiamide) or a H₂-agonist (impromidine) showed that the increase ofL _p induced by histamine was mediated via H₂-receptors.