Mossy fibres, i.e. the axons of dentate granule cells, terminate with characteristic giant boutons on large spines or excrescences of the pyramidal cells in regio inferior of the hippocampus. In addition to pyramidal cells there are several types of non-pyramidal neuron which extend their dendrites into the termination zone of mossy fibres. By using the combined Golgi-electron microscope technique mossy fibre terminals were found, which established asymmetric synaptic contacts with both spines of pyramidal cells and smooth dendrites of identified (Golgi-stained) pyramidal basket cells in the CA3 region of the guinea-pig hippocampus. The observed synaptic connection with pyramidal basket cells suggests an involvement of the mossy fibre system in feed-forward inhibition of the hippocampal pyramidal neurons.