Decamethonium and succinylcholine were used to study the effects of depolarizing muscle relaxants on serum potassium in 60 patients, free of neuromuscular disease, during major orthopedic surgery. Significant increases in serum K+were found after administration of decamethonium or succinylcholine in the usual clinical doses. The abnormal elevations of serum K+found in patients with burns, massive trauma, or muscle denervation are thus acentuations of the process that occurs in normal man following use of these depolarizing drugs. The administration of any depolarizing agent to these abnormal patient groups would, therefore, appear contraindicated.