Simultaneous intracellular recordings were made in the dentate gyrus of rat hippocampal slices, from pairs of the following cell types: granule cells, interneurons located in the granule cell layer, hilar interneurons, and spiny hilar "mossy cells". Granule cells were found to have strong excitatory effects on mossy cells and interneurons. Interneurons inhibited granule cells as well as other interneurons. No synaptic connections from mossy cells onto other cell types were found, within the confines of the slice, using intracellular recording methods. However, at the ultrastructural level, axon terminals of horseradish peroxidase-filled mossy cells were found making synaptic contacts in the hilus on dendrites of interneurons. These studies provide the first step towards determining the functional interactions of the various cell types in the fascia dentata.