The benzylisoquinolinium relaxants currently include the intermediate-acting agent, atracurium, and the short-acting agent, mivacurium. This class of relaxants has always retained the distinct advantages of rapid degradation, enzymatic metabolism, or both in the plasma, resulting in short half-lives and fast, complete recovery, unrelated to dose or duration of administration. Any improvements in this class of relaxants must focus on retaining this property at the same time as decreasing or eliminating histamine release, which has always been the major disadvantage of the benzylisoquinoliniums. The introduction of 51W89, an isomer of atracurium, may represent an advance in the development of this class of relaxants.