1. The extrajunctional spread of end-plate depolarization induced by acetyl) choline (ACh) applied in the bathing solution was studied on frog (Rana esculenta-skeletal muscle fibers. 2. The characteristics of this spread were determined on sartorius muscles mounted in a chamber where the end-plate free pelvic part was separated by a rubber membrane from the end-plate containing tibial one. There was no change in membrane potential if ACh was applied selectively on the pelvic part, even at a concentration of 10 mmol/l. However, on applying 1 mmol/l ACh on the tibial part, depolarization spreading over to the pelvic part was observed. 3. The peak values of ACh depolarization were not modified by tetrodotoxin (TTX) at a concentration of 32 nmol/l, whereas it was decreased by 35% in the presence of 3.1 mumol/l TTX. 4. ACh depolarization measured on the extrajunctional membrane in the presence of TTX is the result of the electronic spread of end-plate depolarization. The length constant of this spread was about the same as the length constant determined by square pulse analysis. 5. When the generation and propagation of action potentials accompanying ACh depolarization were not blocked either by TTX or Na-free Ringer solution, the depolarization values observed on the pelvic part were slightly higher than what could have been expected on the basis of electrotonic behaviour. 6. The above results imply the absence of ACh receptors from the extrajunctional membrane of twitch fibres. Deviations from the electrotonic spread are due to the action potentials accompanying ACh depolarization.