1. The effects of denervation, reinnervation and pH on the ionic permeability changes mediated by junctional receptors have been studied in muscle fibres of the frog sartorius.2. The reversal potential of acetylcholine responses in denervated junctions was about 25 mV more negative than in normal junctions.3. The delay of the change in the ionic properties of junctional receptors was proportional to the nerve stump length: 10 and 14 days for lengths of 12 and 33 mm, respectively.4. When the motor nerve reinnervates the junction, the reversal potential of the acetylcholine responses comes back to the normal value before the neuromuscular transmission is restored.5. The Na/K conductance change decreases in high pH solutions. After denervation, the pH profile of this ratio is shifted to the acid values by about two pH units.6. These observations can be explained by assuming that the Na and K channels differ by the pK of their anionic groups and that the denervation induces an alteration of the sites that bear the charges.