The interaction of propranolol with model phospholipid membranes was studied using various experimental techniques. The partition coefficient of propranolol in the negatively charged membranes of vesicles prepared from phosphatidylserine and phosphatidic acid was found to be more than 20-times higher than in neutral phosphatidylcholine membranes. Preferential interaction of propranolol with acidic phospholipid membranes was confirmed using the monolayer compression isotherm technique and the spin-labelling method. Phosphatidylserine monolayers were markedly expanded even at a relatively low drug concentration (5 . 10(-6) M). In contrast, the effect of propranolol on phosphatidylcholine monolayers was much smaller, being detectable only at a higher concentration of the drug (1 . 10(-4) M). Spin-labeling experiments show that propranolol exerts marked ordering effect on bilayers prepared from acidic phospholipids and does not change the order parameter of phosphatidylcholine membranes. The dependence of the propranolol fluorescence spectrum on the polarity of the solvent allowed us to identify the intercalation region of the drug in the membrane. The fluorophore moiety of propranolol was found to be localized in the lipid polar head groups region of the bilayer. The role of electrostatic and hydrophobic effects in propranolol-membrane interaction is discussed and the effect of propranolol on the ordering of phospholipid bilayers is compared with the effects of other anesthetic-like molecules.