1. Effect of grayanotoxin 1 on the soleus and extensor digitorum longus muscles of rats was studied by the electrophysiological methods.
2. Grayanotoxin 1 has an inhibitory effect on the initiation of action potentials of fibers of both muscles. This toxin causes a gradual decrease of the resting potential and an elevation of the critical firing level.
3. As decrease in resting potential induced by this toxin was always associated with reduction of the effective membrane resistance and these phenomena were not observed upon withdrawal of Na from the medium, depolarization of the membrane in the toxic solution is considered to be due to increased permeability of the membrane to Na ions.