(SN) S U M M A R Y Thirteen mammalian aquaporin (AQP) isoforms have been identified, and they have a unique tissue-specific pattern of expression. AQPs have been documented in the reproductive system of both male and female humans, rats, and mice. However, tissue expression and cellular and subcellular localization of AQPs are unknown in the female reproductive system of pigs. In this study, AQP1 immunoreactivity was detected in the capillary endothelium of the ovary. Distinct immunolabeling of capillary endothelium was also observed in the oviduct and uterus. AQP5 was expressed in flattened follicle cells of primordial follicles, granulosa cells of developing ovarian follicles, and muscle cells of the oviduct and uterus. Staining of AQP5 was also observed in the epithelial cells of the oviduct and uterine epithelium. AQP9 immunoreactivity was observed in granulosa cells of developing follicles. AQP9 was also localized in the luminal epithelial cells of the oviduct and uterine epithelia cells. This is, to our knowledge, the first study that shows tissue expression and cellular and subcellular localization of AQPs in the reproductive system of the female pig. Moreover, these results suggest that several subtypes of the AQPs (AQP1, 5, and 9) are involved in regulation of water homeostasis in the reproductive system of gilts. The first confirmation of AQP in the female reproductive system was obtained by isolating and sequencing the cDNA encoding a water channel from the human uterus (Li et al. 1994). Afterward, Li et al. (1997) found AQP1 mRNA in the rat uterus. To date, based on protein expression, at least nine AQP isoforms have been shown to be expressed in the female reproductive system of humans, rats, and mice (reviewed by Huang et al. 2006). Their specific expression pattern suggests that they play a role in water movement between the intraluminal, interstitial, and capillary compartments. There is also evidence indicating that ovarian steroid can regulate the expression of several AQPs (Jablonski et al. 2003;Branes et al. 2005;Lindsay and Murphy 2006). Nevertheless, data concerning the expression and role of AQPs in female reproductive tissues are still very limited and not available in relation to farm animals.