Astrocytes induce blood–brain barrier (BBB) properties in brain endothelial cells (EC). .O2., generated in blood and EC, opens the BBB. Hence, high activity of superoxide dismutase (SOD) is a prerequisite for normal BBB function. Therefore, the influence of rat astrocytes on the expression of manganese (Mn)SOD in rat EC was investigated in two coculture models of the BBB, allowing either exchange of soluble factors or additionally cellular contacts. Activity, protein content and mRNA expression of endothelial MnSOD were significantly increased in both coculture models in comparison to monoculture by soluble astrocytic factors, such as cytokines. High activity of endothelial MnSOD may be considered as a further essential property of the BBB, which is induced and maintained by astrocytes.