A comparison of the effects of several quaternary ammonium ions has been made on the intracellularly recorded endplate potential of curarized frog muscle. The hydroxyanilinium ions usually caused an increase in endplate potential amplitude, a slowing of rate of rise and marked prolongation, but rarely caused spike generation. Contrariwise, tetraethylammonium and triethylphenylammonium consistently caused an increase in both amplitude and rate of rise of the endplate potential but no prolongation; these endplate potentials generated muscle spikes. The results suggest a relationship between rate of rise of the endplate potential and the probability of spike generation. Using the neurally evoked contractile response as a signal of transmitter action, the hydroxyanilinium ions were found to be relatively weak antagonists of tubocurarine at deep levels of curarization, indicating a ceiling effect. There was also a ceiling to the effect of hydroxyaniliniums on endplate potential amplitude. Thus, in the presence of high concentrations of tubocurarine, tetraethylammonium is a more potent anticurare agent than is triethyl(3-hydroxyphenyl) ammonium.