1. Elevation of the external potassium concentration resulted in a marked positivation of both intracellularly and extracellularly located micro-electrode tips, in intact connectives, with no appreciable reduction in the amplitude of the recorded action potentials.
2. Sucrose-gap experiments showed that this effect was abolished when tension was applied to the nerve and/or if the connectives were briefly exposed to air. In such preparations continuous and relatively slow potential changes were observed, corresponding to depolarization of the giant axons, with only small extracellular positivations of around 5-6 mV.
3. It is concluded that this effect resulted from the access of potassium ions into the extracellular system most probably as a consequence of the disruption of intercellular occlusions at the inner margin of the perineurium, a situation which contrasts with the restricted intercellular penetration associated with preparations exhibiting extracellular positivation. The possible ionic bases of the latter phenomenon are discussed.