Changes of membrane potential (MP), amplitude of action potential (AAP) and continuance of AP (CAP) of identified neurons of leech Haemopis sanguisuga have been investigated at high intracellular Na⁺ concentration. It has been shown that a moderate increase of intracellular Na⁺ concentration leads to hyperpolarization of the membrane to 15 mV and a certain increase of AAP and CAP. A further increase of [Na⁺]in leads to a sharp decrease of AAP and small decrease of CAP. Possible mechanisms of the phenomena described are discussed.