Synaptophysin expression was assessed in dentate gyrus prepared from aged (22 months) and young (4 months) rats by immunoblotting and post-embedding immunolabelling at electron microscope level. Immunoblotting showed qualitatively that there was a marked reduction in synaptophysin expression in synaptosomes of aged compared with young rats. Immunogold labelling studies in the medial molecular layer of the dentate gyrus demonstrated that gold particles were restricted to pre-synaptic boutons, and were present mainly on the membranes of the synaptic vesicles or occasionally inside vesicles. In aged rats, immunolabelling patterns and the density of immunogold particles per pre-synaptic bouton were almost 50% lower than in younger rats. However, because boutons were larger in older rats the actual labelling density per unit area of bouton (3.77) was even lower than in the young rats (7.74). The role of synaptophysin in neural plasticity and ageing should be further examined.