METHODS The technique for intracellular recording of epp's and miniature potentials has been described in earlier papers (Fatt & Katz, 1951, 1952a; Castillo & Katz, 1954). The m. ext. 1. dig. IV of English frogs was used, immersed in an isotonic solution containing concentrations of CaCi2 and MgCI2 adjusted so as to reduce the response to any desired level. In most experiments prostigmine (10-6, w/v) was added to increase the amplitude ofthe potentials (without altering their'quantum content'). The usual procedure was to locate a suitable spot with the internal electrode and record