The aim of the present study was to investigate whether the nucleus reuniens thalami (RE) innervates inhibitory cells in hippocampal field CA1. Therefore, we examined the RE-CA1 input at the ultrastructural level. RE axons were anterogradely labeled with biotin-conjugated dextran amine (BDA), in combination with pre-embedding gamma aminobutyric acid (GABA)-immunolabeling of neurons in CA1. We observed that part of the BDA-labeled axons formed asymmetrical (i.e. excitatory) synapses on GABA-positive dendrites. Based on these data, which are in line with our previously published electrophysiological observations (Dolleman-Van der Weel, M.J., Lopes da Silva, F.H. and Witter, M.P., Nucleus reuniens thalami modulates activity in hippocampal field CA1 through excitatory and inhibitory mechanisms. J. Neurosci., 17 (1997) 5640-5650), we propose that RE-CA1 input partially influences hippocampal transmission through activation of local inhibitory interneurons.