AbstractWe synthesized a family of neuromuscular blocking agents (NMB) based on decamethonium, but containing a carborane cluster in the methylene chain between the two quaternary ammonium groups. The carborane cluster isomers o‐NMB, m‐NMB, and p‐NMB were tested in animals for neuromuscular block and compared with agents used clinically: rocuronium and decamethonium. All three isomers caused reversible muscle weakness in mice as determined by grip strength and inverted screen tests, with a potency rank of p‐NMB > rocuronium > decamethonium > m‐NMB > o‐NMB. The mechanism of action of the compounds was determined by using the in vitro rat phrenic nerve hemi‐diaphragm preparation and electrophysiologic measurements in cells. Neostigmine reversed hemi‐diaphragm weakness caused by the three isomers and rocuronium, but not succinylcholine. In electrophysiologic recordings of currents through acetylcholine receptor channels, the carborane compounds did not activate channel activity but did inhibit channel activation by acetylcholine. These results demonstrate that the carborane neuromuscular blocking agents are non‐depolarizers in contrast to the depolarizing action of the parent compound.