Several authors have shown recently that the neurons in transverse hippocampal slices incubated in the appropriate medium retain all basic electrical characteristics of hippocampal neurons in vivo, which makes this preparation a promising one 20,22,26. One of the most intriguing problems of hippocampal neurophysiology is the problem of structural and functional relations between the dentate fascia (DF) and the regio inferior of hippocampus (fields CA 3–CA 4). Interest in DF is increased by observations of the unusual plasticity of the perforant path synapses upon the granular cell dendrites. This was shown by many authors with in vivo 4,5,17 and in vitro conditions 11−13. Some possible morphological bases for these changes were also suggested 24. In our previous experiments, performed in unanesthetized, slightly restrained rabbits, it was shown that plasticity in the form of long-lasting (hours and days) post-tetanic potentiation (LLP) exists also in another link of the entorhinal cortex-DF-CA 3 chain, namely, in mossy fiber synapses upon CA 3 apical dendrites 6,7. The threshold electrical stimulation of DF brought about, in CA 3 neurons recorded daily in chronic experiments, the following statistically reliable changes: (1) the number of spikes per stimulus (Cr, ‘coefficient of reaction’) increased from day to day; (2) the mean latencies of responses substantially decreased (by 8–10 msec); (3) the duration of post-excitatory inhibition decreased; (4) the range of active frequencies of stimulation widened, so that low frequencies (0.2–3 cycles/sec) began to evoke CA 3 responses. A wide dispersion of latencies was observed in these experiments for the fixed points of stimulation in DF and recording in CA 3. In the experimental conditions used it was possible that various multisynaptic chains (involving commissural, entorhinal and septal connections) were activated by DF stimulation. It was necessary DF synapses upon CA 3 neurons initially have different degrees of modification, obtained in vivo. Conditions which equalize the degree of modification and increase overall synaptic efficacy diminish the scatter of latencies.