In the early 1980s the clinical use of nondepolarizing neuromuscular blocking drugs (muscle relaxants) made a significant leap forward with the introduction of vecuronium and atracurium. These two drugs have an intermediate duration of action, vecuronium is free of cardiovascular effects, and atracurium has a unique non-organ-dependent mode of elimination . Vecuronium and atracurium were a significant improvement over the previously available muscle relaxants, and the usefulness of intermediate-acting drugs became firmly established. Any muscle relaxant introduced since vecuronium and atracurium must have some aspect of its pharmacology that gives it a clinical advantage over these two drugs. In recent years, four new muscle relaxants, pipecuronium (Arduan), doxacurium (Neuromax), mivacurium (Mivacron), and rocuronium (Zemuron) have been introduced into clinical practice in the United States. Other muscle relaxants are at the stage of clinical trials. I will discuss the pharmacology of the drugs recently released into clinical practice, and provide an outline of the drugs undergoing clinical trials.