Muscle relaxants are routinely administered during anesthesia. The residual effects of nondepolarizing muscle relaxants must be reversed by the anesthesia provider prior to extubation of the trachea to ensure that patients can safely protect their own airways and maintain adequate ventilation. Many factors can potentiate the effects of muscle relaxants, and each of these factors must be taken into consideration when postoperative muscle weakness is encountered. Only after the cause of residual neuromuscular weakness is discerned can the appropriate treatment be determined. This article describes the anesthetic management of a patient who experienced postoperative residual muscle weakness secondary to unsuspected hyperkalemia. A discussion of potassium's role in membrane excitability and factors that must be considered in the differential diagnosis of residual neuromuscular weakness is included.