The blood-brain barrier of higher vertebrates is formed by the layer of endothelial cells lining the brain microvessels. The close anatomical association between endothelial cells and perivascular astrocytic end feet suggests cooperation between these cell types in forming and maintaining the blood-brain barrier. This review considers evidence from grafting experiments, developmental studies and culture models of the brain endothelium, concerning the inductive influences acting on the endothelium, and from endothelial cells acting on perivascular astrocytes. Examples from pathology and neurotoxicology which may involve breakdown of induction are also considered.