Immunocytochemical staining for ganglioside GD3 (II3 alpha(Neu Ac alpha 2-8 NeuAc)-LacCer) in the choroid plexus of the adult rat brain was carried out. A peroxidase reaction product for GD3 was predominantly present along the surface of microvilli of the choroidal cells, suggesting that ganglioside GD3 may be concerned with the formation or secretion of the cerebrospinal fluid.