The beta adrenoreceptor antagonist propranolol is rapidly taken up and accumulated in various cultured cell lines. When incubated in the presence of low concentrations of propranolol (10(-9) M), Hela (non-differentiated epithelia), BC3H1 (smooth muscle) and MDCK (differentiated kidney epithelia) cell cultures take up (t1/2 = 4-10 min) and accumulate the drug such that the intracellular concentration is over 1000 times that in the incubation medium. The release of propranolol from the cells was slower (t1/2 = 22 min) than the rate of uptake but the dissociation was stimulated by the addition of 1 microM propranolol to the external medium (t1/2 = 9 min). Uptake, which is non-stereoselective, is dependent on pH and is inhibited by the lysosomotropic agents, NH4Cl, methylamine and chloroquine. At higher concentrations (greater than 10(6) M), uptake is accompanied by a visual swelling of intracellular acidic vesicles staining with acridine orange. These results suggest that propranolol, a basic amphiphilic amine, is accumulated within the lysosomes of these cells. Uptake was confined to these cultured cell systems with no chloroquine-sensitive propranolol uptake, being found in isolated rabbit ventricular myocytes, red blood cells or blood platelets. Although alprenolol and cyanopindolol competed with propranolol for uptake, isoprenaline, adrenaline, noradrenaline, phenylephrine, atenolol, practolol and salbutamol were not effective inhibitors. The possible consequences of this uptake and accumulation of propranolol by certain tissues is discussed in relation to the known actions of the drug, particularly during or after abrupt withdrawal from chronic applications.