The endothelial cells of the blood-brain barrier secrete HCO₃^– ions into the brain. On the present evidence it is likely that the normal secretion occurs via Na⁺-driven HCO₃^– transport into the endothelial cells together with efflux via Cl^–/HCO₃^– exchange [1]. The leading candidate for the efflux transporter is AE2 as mRNA for AE2 is prominently expressed in the primary cultured endothelial cells and AE2 protein is associated with the microvessels in brain slices and is seen in Western blots from the cultured cells [2]. It is well documented that astrocytes are involved in induction and maintenance of various blood-brain barrier features but there are few reports on their influence on ion transporters. We here report an investigation on the effects of astrocytes on endothelial cells employing a method of selective ablation of astrocytes in the brain [3]. This method uses systemic injection of 3-chloropropanediol which produces focal loss of GFAP positive astrocytes in certain brain regions including the inferior colliculus. In this region there was virtually complete loss of GFAP expressing astrocytes from 3 to 6 days after injection. There were marked changes in expression of occludin and claudin 5 from PECAM-1 expressing microvessels over a similar time course and these were reversed soon after repopulation of the region with astrocytes. P-glycoprotein expression also markedly decreased and recovered with a similar time course [4]. The present study follows changes in expression of AE2 protein in microvessels using the same methods. AE2 and the endothelial cell marker PECAM-1 were visualized on microvessels in frozen rat brain sections using fluorescence microscopy. This method included antigen unmasking with 1% SDS [5] as reported previously [2]. PECAM-1 remained clearly expressed throughout the next 28 days, but the distribution of AE2 first became discontinuous and then largely disappeared over 3 days. Recovery was underway at 14 days and appeared to return to normal levels by 28 days coincident with the reappearance of GFAP immunoreactive astrocytes. These in vivo results suggest that with AE2, as with P-glycoprotein, there may be a substantial decrease in expression in brain endothelial cells once these cells are removed from the influence of astrocytes and grown in culture. Hence the previously reporteded transport rates of HCO₃^– in cultured cells may have underestimated the transport rates occurring in vivo.