In the investigations of skeletal muscle of frogs the microelectrode study makes it possible to distinguish the tonic fiber from the kinetic or sensory fiber. To the potential obtainable from tonic fibers Kuffler et al. (1) named a small-nerve junctional potential (S. J. P.). In detail, S. J. P., probably originating at a junction from a small motor axon to the tonic fiber, resembles the end plate potential of twitch fiber because S. J. P. shows no overshoot at action and does not generate the propagatable impulses. Though there is an opinion that it is naturally to think that S. J. P. and twitch fiber spike do not indicate the presence of two different muscle fibers but express the different styles of nervous innervation on the same muscle fiber, the author has carried out the present experiments, assuming the existence of a slow fiber in skeletal muscle of frogs. A duality of response of a muscle fiber is not considered in the course of these experiments of frog muscles. The characteristics of the small-nerve junctional potentials were precisely investigated by Kuffler and Vaughan Williams (2) and by Burke and Ginsborg (3). But there is no report on the effect of various drugs on these S. J. P.'s, so the author has demonstrated the effect of externally applied acetylcholine, vagostigmine, sodium fluoride, monoiodoacetic acid, tetraethylammonium chloride, etc. on S. J. P. in order to know whether the transmission of the small-nerve junction is cholinergic or not and to know whether the S. J. P. and its following after potential are caused by the same process or not.