Distribution of two heparin-binding molecules, pleiotrophin (PTN) and midkine (MK), was investigated by immunohistochemistry in the lingual epithelium of the adult rat. In the lingual epithelium, both PTN- and MK-like immunoreactivities were observed in its basal cell layers, showing a mesh-like appearance. These molecules were also found along the surface of the taste bud cells; an intense immunoreaction was detected at the base of the taste buds in the circumvallate and foliate papillae. At the electron microscope level, the immunoreactive products were localized on the cell surface and basement membrane at the base of the taste buds. The immunoreactivity for PTN was distributed more diffusely than that for MK. It was suggested that these molecules may be involved in the differentiation and maintenance of taste bud cells, possible via their trophic effect upon approaching nerves.