1. The effects of quinine and quinidine on the volume-activated chloride current (ICl(vol)) in cultured endothelial cells from bovine pulmonary artery were studied by use of the whole-cell patch-clamp technique. 2. At pH 7.4 both quinine and quinidine induced a fast and reversible block of ICl(vol)) with Ki values of 20 +/- 4 microM and 30 +/- 10 microM, respectively. 3. The blocking efficiency of both drugs increased dramatically with increasing extracellular pH, indicating that the blockade is mediated by the uncharged form of quinine and quinidine. 4. These results suggest a hydrophobic interaction with high affinity between volume-activated chloride channels and uncharged quinine and quinidine within the membrane bilayer of endothelial cells.