Unit responses to stimulation of septo-fimbrial area or fascia dentata were recorded in hippocampus of unanaesthetized rabbits with extra- and intracellular microelectrodes. Neuronal discharge index was found to increase or latency to decrease in 10 out of 28 neurones recorded as long as 3 min to 2.5 hours after tetanization (20/sec, 10 sec). Posttetanic increase in amplitude of excitatory postsynaptic potentials (EPSP) was fo-nd in 8 of 24 neurones recorded intra- and quasi-intracellularly. EPSP latencies were as short as 2 msec in two of these neurones suggesting the monosynaptic origin of the EPSPs. Decrease in excitability was found shortly after tetanization in two neurones tested with intracellular current injections. A tendency to long-lasting posttetanic hyperpolarizing shifts was noted. Thus a long-lasting post-tetanic potentiation (PTP) was revealed at the level of hippocampal single neurone responses. Intracellular analysis suggests an increase in effectiveness of excitatory synapses as a main mechanism of the PTP. No significant adequate changes at the postsynaptic level were found to explain the long-lasting hippocampal PTP.