Carbachol, when applied to the bathing Ringer solution of frog sartorius muscles, caused depolarization of the endplate and a blockade of endplate potentials (EPP's), miniature EPP's (mepp's) and the iontophoretic acetylcholine potential. In muscles treated with an analog of hemicholinium-3, alpha, alpha'bis(dimethylammonium acetaldehyde diethylacetal)-p-p'-diacetylbiphenyl dibromide (DMAE), depolarization of the endplate by carbachol was blocked and the blockade by carbachol of the iontophoretic acetylcholine potential was prevented. These responses to carbachol were attributed to a postjunctional action that was antagonized by DMAE. In contrast, the blockade by carbachol of EPP's and mepp's was enhanced in DMAE-treated muscles at a time when carbachol-induced depolarization was blocked. This response to carbachol was attributed to a pre-junctional action. Carbachol either blocked transmitter release by a mechanism that was insensitive to DMAE or enhanced the prejunctional blocking actions of DMAE. Succinylcholine had actions similar to carbachol. DMAE prevented depolarization by succinylcholine but enhanced neuromuscular blockade by succinylcholine. SKF 525-A (beta-diethylaminoethyl diphenylpropylacetate hydrochloride), like DMAE, prevented depolarization but not transmission blockade caused by carbachol.