Anesthesiologists perceive that the ideal muscle relaxant is not yet available, particularly the nondepolarizing one with a rapid onset and a short duration of action. There is also a need for relaxants with different durations of action but which would be free from side effects. During the process of this development several new compounds have been tested and four have reached an advanced state of study; three of these, doxacurium, pipecuronium, and mivacurium are already licensed and rocuronium is likely to be licensed in the near future. Doxacurium and pipecuronium are slow onset and long duration of action compounds but singularly free from cardiovascular side effects. Mivacurium has an onset comparable to that of atracurium and vecuronium but with a duration of action which is intermediate in duration between these drugs and succinylcholine. Rocuronium is a drug with a fast onset of action capable of being used in place of succinylcholine but with a duration of action which is similar to that of vecuronium.