1. The consequences of stimulating a single pyramidal cell in the CA1 area of the hippocampus for synaptic transmission in the stratum radiatum were investigated. 2. Tetanic activation of single pyramids caused by depolarizing current injection, but not an equal number of distributed action potentials, reduced excitatory transmission by 20 %, with a delayed onset, for more than 1 h. 3. EPSPs in the tetanized pyramidal cells were increased for equally long periods but this was not the cause of the field EPSP reduction. Spontaneous somatic IPSPs were not affected; evoked IPSPs were decreased in the tetanized cell. 4. Paired pulse facilitation of the field EPSPs was unchanged. 5. The field EPSP reduction was markedly diminished by a knife cut along the base of pyramidal cells in CA1. 6. The addition of antagonists of GABA, NMDA and metabotropic glutamate receptors blocked or diminished the field EPSP slope reduction evoked by intracellular stimulation. 7. Simultaneous recordings revealed long-lasting excitations of interneurons located in the outer oriens layer as a result of single pyramid tetanization. 8. Intense firing of small numbers of pyramidal cells can thus persistently inhibit mass transmission through the hippocampus. This effect involves activation of interneurons by glutamate receptors.