Recently, hyperpotassemia was reported in patients treated with propranolol, but the mechanism has not yet been delineated. We have investigated the effects of various βadrenergic blockers and of D-propranolol on the Na+ and K+ distribution intra- and extracellularly in human erythrocytes. K+ loss and Na+ gain by cells was demonstrated at drug concentrations of 10-4 M or greater. D-Propranolol was more effective than L-propranolol, whereas pindolol was ineffective. Practolol increased Na+ content but did not influence K+ The results suggest that electrolyte redistribution across cell membranes is not a likely explanation for hyperpotassemia in patients treated with propranolol, or for the local anaesthetic effect of this drug.