Hydrocortisone is known to induce barrier properties in porcine primary cultures of microvascular endothelial cells. Here we present similar effects of hydrocortisone on a serum-free in vitro model based on primary cultured mouse brain endothelial cells. These cells in culture express typical blood-brain barrier properties and the transendothelial electrical resistance is enhanced after the addition of hydrocortisone to the medium in physiological concentrations. The improvement of the barrier is accompanied by changes at the cell borders indicated by immunofluorescence staining of tight junction proteins. Transmission electron microscopy imaging indicates morphological changes at the cell-cell contact zones which correlates to the observed changes in the transendothelial electrical resistance after HC supplementation. Phalloidin staining of F-actin shows a rearrangement to "fiber-like" structures in the longitudinal direction of the cell. These findings together with additional electrical impedance analysis of the monolayer suggest that several changes including cell-cell contact alteration, cell-substrate attachment and cytoskeletal rearrangements cause enhanced barrier properties in this murine endothelial culture. The present data are consistent with earlier findings in a porcine serum-free in vitro model. Thus, evidence is given that the barrier enforcement induced by glucocorticoids is not a species-specific effect and that the barrier improvement is correlated with a change of the cell morphology rather than changes in tight junction protein expression.