<p>The posterior lobe (PL) of the vertebrate pituitary is richly vascularized with a dense network of fenestrated capillaries. In this study, we found that the expression of <i>Aqp1</i>, which encodes a plasma membrane-localized water channel protein, was significantly higher in endothelial fractions isolated from the rat PL than in those isolated from the anterior lobe (AL). Immunohistochemistry revealed aquaporin 1 (AQP1)-positive signals in fenestrated endothelial cells of the PL. Furthermore, immunoelectron microscopy demonstrated the presence of AQP1 signals on both the luminal and abluminal plasma membranes of these cells. AQP1 plays a pivotal role in facilitating water movement across the plasma membrane in response to changes in osmotic pressure on a cell. To investigate the effect of hyperosmolarity, we examined the expression levels of <i>Aqp1</i> in the PL of water-deprived rats as well as in isolated endothelial cells of the PL cultured in a hyperosmotic medium supplemented with raffinose. Immunohistochemical analysis showed no changes in the proportion of AQP1-positive endothelial cells or in subcellular localization of AQP1 in cultured endothelial cells of the PL under hyperosmotic conditions. In contrast, analysis using quantitative real-time PCR revealed that hyperosmolar conditions significantly downregulated <i>Aqp1</i> expression in cultured endothelial cells. These findings suggest that <i>Aqp1</i>expression in fenestrated capillaries in the PL is regulated by osmotic pressure of the interstitial fluid. Our results indicate that AQP1 is selectively expressed in fenestrated capillaries of the PL and plays a crucial role in maintaining water homeostasis in this region.</p>