The choroid plexus of the rat was examined immunocytochemically using both an antibody directed towards a histamine thyroglobulin conjugate as well as to histidine decarboxylase. Histamine- as well as histidine decarboxylase-immunoreactive cells were found within or in the vicinity of the plexus epithelium, with processes extending to the neighbouring epithelial cells or surrounding tissue, suggesting a specific function for histamine on the choroid plexus.