Amongst the six aquaporins (AQPs) presently found in mammalian salivary glands, AQP5 seems to be the one most important for salivary fluid secretion. Indeed, one finds a markedly decreased saliva production in aquaporin 5 knock-out mice (1). Altered cellular aquaporin 5 localization may also play a role in Sjögren’s syndrome, a complex autoimmune disease associated with reduced salivary and lacrimal flow (2). We study the presence and localization of aquaporins, in particular AQP5, in salivary gland tissue during foetal and postnatal development. We wish not only to learn more of its role in saliva production in mature glands, but also if the aquaporins could influence glandular development itself, as has been seen with AQP11 in mouse kidneys (3). The presence of AQP5 has been documented pre- and postnatally in the mouse submandibular gland, but AQP5 has also been found in the closely associated sublingual gland (SLG) in adult mice. Its immunolocalization is to our knowledge not yet described in the gland during foetal development. Therefore, in this study we wanted to examine the immunolocalization of AQP5 in the mouse SLG during development and in the adult gland. Immunohistochemistry was performed on paraffin embedded salivary glands from embryonic day 16.5 (E16.5) and E19.5 (day of birth) as well as from adult males and females (postnatal day 60). AQP5 was detected with anti-AQP5 antibody (Alomone labs) using the Ventana Discovery XT (Ventana Medical System). At E16.5, AQP5 was found in a scattered pattern throughout the terminal tubules. Cytoplasmic staining was found in the positive cells. No staining could be found in the presumptive ducts at this age. At birth (E19.5), all proacinar cells were AQP5 positive. The staining showed presence of AQP5 in the apical, lateral and – although less prominent – basal membrane. AQP5 was also localized to the apical membrane in the juxta-acinar duct segment, while the remaining ducts were all negative. In the adult (P60) salivary gland acini, AQP5 was mostly localized to the luminal membranes. Some staining was also found in the basal membrane. In the following duct segment, the intercalated duct, AQP5 was only observed in the apical membrane. The remaining duct segments showed no AQP5 expression. There was no apparent difference between the sexes. Previous staining of AQP5 in acini of the human SLG showed a similar distribution pattern in the luminal membranes (4). Our novel results of AQP5 immunolocalization in the embryonic SLG show a more spread pattern of activity that gradually becomes more similar to that found in the adult glands. By establishing when and where the important salivary aquaporins appear, we hope to gain insight into their potential role in proper organ development.