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The electrophysiological properties of hippocampal slices from trained and untrained rats presented with identical numbers of combinations during acquisition of a conditioned two-sided avoidance reaction were compared. Associative learning (presentation of combinations of conditioned and unconditioned stimuli) was accompanied by gradual increases in the amplitude of the population spike in hippocampal field CA1. However, achievement of the learning criterion was accompanied by a decrease in the amplitude of the population spike. The same type of difference between groups was seen in field CA1 using low-intensity stimulation of Schaffer collaterals, while there was no change in the amplitude at maximum-intensity stimulation. After the rats achieved the learning criterion, the level of facilitation in conditions of paired stimuli was significantly greater than in untrained animals and also as compared with the passive and active controls. These data provide evidence supporting the hypothesis that changes in the efficiency of synaptic transmission occur during the learning process. However, the direction of changes depends on the extent of training.