administered intraventricularly are taken up by granular pericytes in cerebral fine vessels. These cells are provided with elongated and complicated infoldings. Pinocytotic vesicles occur often along the infoldings, and ends of infoldings are swollen and assume vesicular shapes. The specific morphology of the granular perithelial cells is considered t o favor a removal of metabolic wates in cerebral tissue.Exogenous substances such as trypan blue and horseradish peroxidase It is known that systemically injected trypan blue and horseradish peroxidase d o not penetrate the vascular wall o f cerebral vessels because of the blood-brain barrier. The tracers do, however, spread into cerebral tissues if they are administered intraventricularly. The removal of exogenous molecules from cerebral tissues has scarcely been explored.in the removal of exogenous tracers. MATERIAL AND METHODSWe report here experiments indicating that granule-containing vascular pericytes are involvedThe animals employed in this study were Wistar rats, aged 4-5 months. They were bred in the authors' laboratory and fed on a standard rat chow (CLEA Co. L.T.D., Tokyo). They were divided into two groups (A and B).into the left cerebral ventricle. In the group-B rats, the same volume of physiological saline containing 2 mg horseradish peroxidase was administered similarly.At 60 minutes after the injection, the rats were killed by decapitation. The brains were removed and placed into cold physiological saline. Under a dissecting microscope, the cerebral cortex on the side opposite the injection was sliced with a 1-azor blade. Some sliced specimens were used as stretch-specimens for light microscopic observations (Mato and Ookawara, '79). They were stained with hematoxylin-eosin, PAS (periodic acidschiff reaction) or Sudan black B. The others were immersed in a solution containing 2% paraformaldehyde and 2.5% glutaraldehyde, buffered with 0.1 M phosphate buffer (pH 7.4) for 4 hours. The specimens of group A were postfixed with 1% osmium tetroxide in 0.1 M phosphate buffer for 2 hours. The specimens of group B were again washed with 0.1 M phosphate buffer overnight, and treated with 0.03% dianiinobenzidine and 0.01% hydrogen peroxide solution for 10 minutes. The Specimens were then postfixed in the same manner as those froin group A. Standard procedures were used for embedding and cutting. To observe the uptake of tracer in the light microscope, stretch-specimens were stained with eosin for the trypan-blue group, and for horseradish peroxidase, stretch-specimens were treated with 0.03% diaminobenzidine and 0.01% hydrogen peroxide for 6 minutes.In the group-A rats, 0.05 ml of 1% trypan blue in physiological saline solution was injected