Evidence is presented that long-lasting enhancement of perforant path synapses following high-frequency activity is a cooperative process requiring coactivity of a considerable number of fibers. This was demonstrated in two ways: (1) A threshold stimulus intensity during high-frequency stimulation was required before any enhancement was observed. This threshold was considerably above the stimulus threshold for observing a minimal synaptic response in the fascia dentata. Above the enhancement threshold, the amount of enhancement increased as the intensity of high-frequency stimulation increased. (2) Concurrent high-frequency activation of fibres of the medial and lateral perforant path produced enhancement at intensities where identical but independent stimulation of the two pathways either failed or was less effective. Discharge of the granule cells during the high-frequency stimulation was independently manipulated and found not to be physically coupled to the enhancement generating mechanism.