We investigated the presence of Ca2+-activated Cl- channels in adult rat alveolar type II (ATII) using patch-clamp techniques. Only one active channel each, with a single channel conductance of 50 pS and an opening probability (Po) of 0.76 was found among 130 successful cell-attached and 5 inside-out patches. Addition of CPT-cAMP into the bath (500 μM) induced one active patch from 33 silent cell-attached patches. Incubation of 9 ATII cells, with ionomycin (1 μM), failed to elicit chloride single currents in 9 cell-attached patches. Cl- currents were also absent from 35 whole cell patches, even after the addition of 10 μM terbutaline in the bath or 1 mM ATP and 5 mM MgCl2 in the pipette. These results indicate that only a very small fraction of adult rat ATII cells express CFTR and suggest that Cl- ions are passively transported across the cell junctions. © 1996 Academic Press, Inc.