The granule laden cells located around fine vessels in Wistar rat cerebral cortex were investigated with fluorescent, light microscopy and EM. These cells were designated as fluorescent granular perithelial cells (FGP). The FGP were one of essential components of cerebral vessels and separated clearly from nervous tissue by a basal lamina. They were often observed at bifurcating regions of fine vessels. The FGP seemed to differentiate morphologically around birth, and the specific granules in them began to emit yellow fluorescence at the 2nd week and attained to a definite level of fluorescence intensity at the 8th week. Microfluorometrically, the peak of emission stood at 530 .mu.m. Accompanying growth of the animals, eosinophilia of granules became strong. Sudanophilic areas in the FGP were limited to a periphery of granules. Anisogranularity was evident in old rats, aged 2nd and 3rd year. By EM, the specific cytoplasmic organlles in the FGP were round electron opaque bodies and vesico-tubular system or sac shaped smooth surfaced endoplasmic reticula. The electron opaque bodies appeared clearly at 2nd week after birth, and changed their profiles and contents with aging of rats. In old rats, they looked porous and honeycomb like structure, and sometimes, whole cytoplasm of the FGP was occupied with these structures. Some sac shaped endoplasmic reticula contained less intense material in 1 side and seemed to be associated with a formation of electron opaque bodies. The extensions of some FGP encircled completely the entire circumferences of cerebral vessels. It looked as if fine vessels would penetrate into the FGP. After administration of major tranquilizers and prednisolone, fluorescency and eosinophilia of intracellular granules decreased moderately; vitamin E and B enhanced both. 5 HT [5-hydroxytryptamine] and 5 HTP [5-hydroxytryptophan] enhanced the fluorescency, but suppressed the eosinophilia. From the findings mentioned above, the FGP were expected to play an important role in a transport of nourish substance from fine vessels to neurons, and in a removal of some products in the extracellular spaces of CNS.