Many of the hormone-regulated ion transport processes in distal nephron involve transcellular pathways which require a passive entry of ions at the apical membrane of the distal tubule cells. To investigate molecular mechanisms underlying the ionic permeability of the distal tubule apical membrane, a study was undertaken in which vesicles prepared from apical membranes from isolated rabbit distal tubules were fused onto a planar lipid bilayer. These experiments led to the identification of several ionic channels including a Cl - -permeable channel of 14 pS with a Na + over Cl - permeability ratio,P N a P C l < 0.09. The open channel probability (P o ) showed a weak voltage dependency with P o increasing slightly at negative potential values (intracellular (trans) relative to extracellular (cis) for right-side-out vesicles). Channel activity was inhibited by NPPB at high concentrations (\gr 100 μM) and by DIDS (300 μM). A small inhibitory effect was also observed in the presence of DPC at concentrations ranging from 200 μM to 500 μM. The presence of SO 2 - 4 (32 mmol/l) in the trans solution caused a complete inhibition of channel activity, but no modification of channel behaviour was observed with the non-selective channel blocking agent gadolinium (Gd 3 + ) at 100 μM. Finally, addition of the catalytic subunit of protein kinase A into the trans chamber (60 U/ml to 80 U/ml) led to an increase in channel activity characterized by a greater number of active channels coupled to an increase of the individual channel open probability. The action of the protein kinase A could be cancelled by the addition of a non specific protein phosphatase, such as alkaline phosphatase. Our results suggest that the apical membrane of the rabbit distal tubule contains a Cl - permeable channel of small conductance the activity of which may be modulated by hormones linked to the adenylate cyclase pathway.