'Perineuronal nets' are ill-known structures enwrapping the cell bodies and proximal dendrites of certain neurons in the brain. It is as yet unclear if they represent a cytological entity or extracellular material. Using immunohistochemical methods we have detected the presence of the extracellular matrix-protein, tenascin, in the 'perineuronal nets' surrounding certain cortical interneurons. We have also shown that tenascin antibodies label the circumference of parvalbumin-immunoreactive neurons preferentially. We conclude that this classical matrix protein is a major component of 'perineuronal nets'. Therefore, 'perineuronal nets' may represent sites of privileged adhesion between nerve and glial cells.