The perturbation effect of the beta-adrenoceptor blocking drugs atenolol, propranolol, practolol, oxprenolol, doberol, pronethanol, metipranolol, alprenolol, Kö-1124, pindolol, and exaprolol on rat brain lipid membrane was investigated by ESR spectroscopy using the spin probe method. Using stearic acids spin labeled at the 5th, 12th, and 16th positions, it was found that lipophilic drugs disorder the membrane and their effect is about 5-10 times higher at the 16th carbon membrane depth than at the 5th depth. Exaprolol induced nonlamellar phases in the bovine brain lipid membrane as detected by 31P NMR spectroscopy. The relative potencies of the drugs at 10 mmol/liter concentration to disorder the lipid membrane at the 16th carbon depth were in the order: exaprolol greater than alprenolol approximately equal to propranolol greater than metipranolol approximately equal to doberol greater than control sample greater than pindolol approximately equal to practolol approximately equal to atenolol. This order qualitatively corresponds with some of their nonspecific biological membrane activities but is not related to their beta-adrenoceptor blocking potencies. The inequality of the membrane perturbation propensities of the drugs indicates that they perturb the lipid membrane in a structure-dependent manner, i.e., that each induces a specific rather than a nonspecific membrane perturbation.