The presence of the glial marker proteins, the S-100 and the glial fibrillary acidic (GFA) protein, in the pineal gland was investigated in the rat. Using both the indirect peroxidase-labelled immunoglobulin technique and the unlabelled antibody enzyme (PAP) method, we observed few scattered S-100 and GFA positive cells in the pineal. The number, location and morphology of these cells suggest they are the pineal interstitial cells. This indicates that the interstitial cells are of neuroectodermal origin, possibly macroglial cells themselves.