Aquaporins (AQP), a family of water channels expressed in epithelial cells, function to transport water in a bidirectional manner to facilitate transepithelial fluid absorption and secretion. Additionally, AQP1 and AQP5 are found in pancreatic zymogen granules and synaptic vesicles and are involved in vesicle swelling and exocytosis in exocrine cells and neurons. Here, we show AQP1 is in dense core secretory granule (DCSG) membranes of endocrine tissue: pituitary and adrenal medulla. The need for AQP1 in endocrine cell function was examined by stable transfection of AQP1 antisense RNA into AtT20 cells, a pituitary cell line, to down-regulate AQP1 expression. These AQP1 deficient cells showed >60% depletion of DCSGs and significantly decreased DCSG protein levels, including pro-opiomelanocotin (POMC/proACTH) and prohormone convertase 1/3, but not non-DCSG proteins. Pulse-chase studies revealed that while DCSG protein synthesis was unaffected, 50% of the newly synthesized POMC was degraded within 1 hour. Low levels of ACTH were released upon stimulation indicating that the small number of DCSGs that were made in the presence of the residual AQP1 were functionally competent for exocytosis. Analysis of anterior pituitaries from AQP1 KO mice showed reduced PC1/3, CPE and ACTH levels compared to WT mice demonstrating that our results observed in AtT20 cells can be extended to the animal model. Thus AQP1 is important for maintaining DCSG biogenesis and normal levels of hormone secretion in pituitary endocrine cells.