Hexafluorenium had no effect on the twitch response in anesthetized man. The phase 1 depolarizing or phase 2 nondepolarizing block produced by succinylcholine was increased by the injection of hexafluorenium. Edrophonium under similar circumstances increased the phase 1 depolarizing block, but antagonized the phase 2 nondepolarizing block. The phase 1 depolarizing block produced by decamethonium or Imbretil was antagonized by hexafluorenium, while the phase 2 nondepolarizing block produced by decamethonium or Imbretil was potentiated by hexafluorenium. Edrophonium potentiated the phase 1 depolarizing block of decamethonium and Imbretil and antagonized the phase 2 nondepolarizing block of decamethonium and ImbretiL The d-tubocurarine block was potentiated by hexafluorenium and antagonized by edrophonium. The results with edrophonium were attributed to inhibition of true cholin-esterase. The effects of hexafluorenium were attributed to its junctional membrane, nondepolarizing and pseudocholinesterase inhibiting actions.