Immunocytochemical studies using a monoclonal antibody to choline acetyltransferase (ChAT) were performed on sections of rat dentate gyrus. Light microscopical analysis of the immunoreactivity revealed dense fiber networks and many punctate structures predominantly located at the interface of the granule cell layer and molecular layer. In the electron microscope, the immunostained punctate structures were identified as synaptic boutons which formed mainly symmetrical contacts onto dendritic elements. Few ChAT-immunoreactive boutons formed axosomatic contacts.