Cardiac arrhythmias follow -ing administration of a second dose of succinylcholine was produced in patients anesthetized with halothane. These arrhythmias were not seen when anesthesia was induced with a sleep dose of thiopental, or when the patients were first given a dose of hexafluorenium. Evidence is presented to support the hypothesis that the patient is "sensitized" to the second injection of succinylcholine by the choline formed in the hydrolysis of the first injection of succinyldicholine and that the "sensitized" patient then reacts to the unhydrolyzed succinyldicholine molecule.