Vascular endothelial cells (EC) exhibit organ-to-organ heterogeneity in their functions and morphologies. In particular, brain capillary EC have unique characteristics exemplified by the blood-brain barrier (BBB). The formation and the maintenance of BBB have been ascribed to EC responses to inductive signal (s) or factor (s) from astrocytes that encircle microvessels in the central nervous system. These EC responses were demonstrated in numerous in vivo studies, exemplified by those of Janzer and Raff (Nature 325: 253, 1987) and Tout et al.(Neuroscience 55: 291, 1993) showing that transplanted astrocytes induced BBB properties in non-neural vascular EC. In this study, we constructed a heterologous co-culture system, in which rat fetal brain astrocytes were cultivated on one surface of a porous membrane and human umbilical vein EC on the opposite surface. Electron microscopic examination revealed that astrocytes passed their endfeet through the pores, making contact with EC. In this system, γ-glutamyltranspeptidase (γ-GTP) activity in EC was found to be significantly increased by contacting astrocytes in a density-and time-dependent manner, but not when the astrocyte feeder layer was apart from EC or replaced by COS cells; astrocyte-derived extracellular matrix partially activated γ-GTP. mRNAs for some of the representative BBB markers, including transferrin receptor, P-glycoprotein, brain-type glucose transporter (GLUT-1), and γ-GTP were also demonstrated by reverse transcription-polymerase chain reaction to be upregulated in EC co-cultured with astrocytes. Astrocyte inductions of close membrane apposition resembling a zonula occludens and of an increase in the content of mitochondria in EC were also noted in electron micrographs. Furthermore, an increased barrier activity against inulin was conferred on EC when they were lined with astrocytes. The results obtained with this heterologous co-culture system thus indicate that through contact with their feet, astrocytes are capable of transdifferentiating non-neural EC into the brain type, endowing them with the BBB properties. Glia 19: 13–26, 1997© 1997 Wiley-Liss, Inc.