In this study we show that the aminopeptidase N of cerebral pericytes (pAPN) associated with the blood-brain barrier (BBB) is downregulated in pericytic cell cultures. This observation is in accordance with previous data describing comparable in vitro effects for BBB-specific enzymes of endothelial or pericytic origin, such as -glutamyl transpeptidase or alkaline phosphatase. By polymerase chain reaction and in situ hybridization we were able to determine that the down-regulation of pAPN occurs at the posttranscriptional level. The mRNA of pAPN was found to be constitutively expressed even when the protein is no longer detectable. Culturing the pericytes in an endothelial cell-conditioned medium allowed pAPN to be reexpressed. However, the reexpression effect depended largely on the culturing conditions of the pericytes. Although purified pericytes deprived of endothelial cells did not reveal a reexpression effect, pericytes that were kept in contact with endothelial cells were able to acquire a pAPN-positive phenotype, indicating that endothelial cells constitute an essential requirement for the in vitro reexpression of pAPN. Astrocytes, however, were insufficient in exerting any reexpression effect.