1. An analysis has been made of the distribution and rise time of the responses induced by electrophoretic application of ACh on junctional and extrajunctional receptors of the neuromuscular junction of the frog.2. The junctional and the extrajunctional responses appear to constitute two independent groups with no overlapping characteristics: (a) the responses evoked from junctional receptors show always a high ;sensitivity' (30-100 mV/nC) and a rise time usually shorter than 10 msec that does not lengthen as the ACh dose is increased; (b) the responses evoked from extrajunctional area show a lower sensitivity (0.5-10 mV/nC) and a slow rise time that is a linear function of the dose Q((2/3)) of ACh.3. The lengthening of the rise time of extrajunctional response with increasing doses of ACh is interpreted by supposing that larger doses saturate larger areas of a membrane of low sensitivity uniformly covered with receptors. On the contrary, at junctional spots the does of ACh would be insufficient to saturate the receptors and rise time will be independent of dose.4. Relatively unresponsive zones may exist between junctional spots and extrajunctional areas and between neighbouring extrajunctional areas.5. Extrajunctional areas are subjected to significant seasonal variations; in summer they are practically absent, in autumn they appear as elongated areas of about 20 mu wide and 50 mu long, and late in winter they fuse together and all the ;end-plate' zone becomes sensitive to ACh.