The volume of the fluid in the female reproductive tract fluctuates during the oestrus cycle suggesting that water availability is under hormonal control. Water movement across epithelial barriers occurs through water channels formed by aquaporins (AQPs). We have previously demonstrated that rat AQP9 levels are increased during the periovulatory period of the reproductive cycle in an oestrogen- and progesterone-dependent manner (1). Now we have explored the functionality of these channels in fluid formation and the possible AQPs involved in water movement through the rat uterine epithelium. Rats used in all experiments were anaesthetized intraperitoneally with sodium pentobarbital (55 mg/kg weight) before any experimental procedure including humane killing. The instillation of uterine horns from rats in proestrus with 100 μl of 200 μM HgCl₂ produced 6 h later a 52.4 ± 18.6% (p<0.05) reduction in the fluid volume compared to that of horns instilled with 0.9% NaCl. Using RT-PCR of total RNA from uterine epithelium of rats in proestrus, we identified AQP2, -3 and -9. By immunoblotting of total membrane homogenates we detected a 29 kDa band for AQP2 and a 52 kDa band for AQP9, suggesting that this epithelium expresses a modified form of AQP9. Both AQPs were detected during the whole oestrus cycle. While levels of AQP2 were relatively constant, those of AQP9 increased mainly during oestrus and less prominently during proestrus (0.205 ± 0.020, 0.108 ± 0.022, 0.029 ± 0.004 and 0.017 ± 0.003 correspond to AQP9 relative levels in oestrus, proestrus, diestrus 1 and 2, respectively, n=3). Immunohistochemical studies of AQP9 in uterine sections showed positive reaction only in glandular and luminal epithelial cells. Our results suggest that water channels formed, at least by AQP2 and -9 may mediate water movement across this epithelial barrier and that up regulation of AQP9 may account for an increase in uterine epithelial membrane water permeability. Further experiments are in progress to evaluate the localization of AQP2 and -9 in basolateral or apical membrane as well as to determine if other AQPs are also present in this epithelium.