Integrins are a large family of cell adhesion receptors mediating cell–extracellular matrix (ECM) interactions and are widely distributed in tissues. The β8 integrin subunit mRNA has been shown to be expressed at higher levels in the central nervous system (CNS) than in other organs [M. Moyle, M.A. Napier, J.W. McLean, Cloning and expression of a divergent integrin subunit β8, J. Biol. Chem. 266 (29) (1991) 19650–19658] but its cellular and subcellular localization in the CNS are unknown. In this report, we demonstrate that β8 pairs exclusively with the αv subunit in the CNS to form the αvβ8 heterodimer. Immunohistochemical analysis of the distribution of β8 in adult mouse and rat brains revealed that the protein is expressed in several regions of the hippocampal formation and in the molecular layer and glomeruli of the granular cell layer of the cerebellum. Punctate and diffuse immunolabeling was observed occasionally surrounding neuronal pericarya and extensively throughout dendritic fields suggesting both pre- and post-synaptic localization and/or expression in non-neuronal cells. By immunoelectron microscopy, β8 immunoreactivity was detected in dendritic spines where it was often localized at post-synaptic densities, occasionally in axon terminals and in glial processes. Association of β8 with synaptic membranes was further supported by its enrichment in synaptosomal preparations as detected by immunoblotting. These results demonstrate that αvβ8 is present in mature synapses and therefore may play a role in synaptic function.