1. Experiments carried out in situ, using suction electrodes, and in vitro, using the ‘sucrose-gap’, have demonstrated a restricted access of sodium and potassium ions to the axon surfaces in crural nerves of the cockroach Periplaneta americana and the locust Schistocerca gregaria.
2. Elevation of the external potassium concentration produced appreciable extra-neuronal potential changes in intact crural nerves of the locust.
3. In the locust the presence of the over lying fat body sheath was found to alter the time course of potassium-induced d.c. potential changes.
4. In particular, an initial lag period in the d.c. response is described and tentatively interpreted in terms of a cation reservoir effect.