The mode of action of propranolol, chlorpromazine, and quinine, three cationic drugs inhibiting swelling of yeast mitochondria in potassium acetate, was investigated by looking at their effect on fluorescent probes of the polar heads and of the nonpolar moiety of the membranes, under inhibitory conditions of swelling. As expected, propranolol and chlorpromazine exhibited specificity for anionic phospholipids since they increased the binding of the anionic probe 1-anilino 8-naphthalenesulfonate (ANS). Although propranolol did not release 1,6-diphenyl-1,3,5-hexatriene (DPH) from the hydrophobic moiety of the membrane, it increased the excimer/monomer fluorescence ratio of 10-(1-pyrene)decanoate, suggesting that it induced a limitation in the movements of the aliphatic chains of phospholipids. Opposite to propranolol, chlorpromazine removed DPH from the membrane, suggesting that it bound essentially to the hydrophobic moiety. However, chloramphenicol, which was also able to remove DPH but did not increase the binding of ANS, did not inhibit swelling. Inhibition by chlorpromazine therefore appeared to be related to its binding to the hydrophobic moiety of anionic phospholipids. Quinine had no effect on membrane properties: at inhibitory concentrations of swelling in potassium acetate, it did not inhibit swelling in ammonium phosphate (mediated by the phosphate/H+ cotransporter), whereas propranolol and chlorpromazine did, suggesting a more specific effect of quinine on (a) protein(s) involved in the K+/H+ exchange. Dicyclohexylcarbodiimide (DCCD), which irreversibly inhibits the swelling in potassium acetate, bound to ethanolamine heads; despite this effect, DCCD had no major consequences on the binding of the probes. Consequently, propranolol and chlorpromazine are of no help for characterizing protein(s) catalyzing the K+/H+ exchange, although their effect on lipids seems to involve limited zones of the inner mitochondrial membrane. Quinine and DCCD, although they also bind to lipids, may inhibit the activity by acting on a limited number of proteins.