Estrogen receptor alpha (ER alpha) is an important regulator of the estrous cycle and mice with global ER alpha deletion, as well as some conditional knockout mouse lines, have an interruption in the estrous cycle. In this study we observed that conditional ERa knockout mice where the Cre gene is regulated by the rat insulin promoter (RIP), RIP-Cre/ER alpha(KO) mice, have a 3.7-fold increase in serum 17 beta-estradiol levels, blocked estrous cycle, and develop a fluid-filled uterus (hydrometra). Using a proteomics approach, we identified three proteins, lactoferrin, complement C3 and chitinase 3-like protein 1 (CHI3L1), as highly expressed proteins in hydrometra fluid. The mRNA levels of the corresponding genes were more than 50-fold higher in RIP-Cre/ER alpha(KO) uterus compared to controls. High expression of CHI3L1 in the uterine fluid was not reflected as elevated levels in the serum. The high expression of lactoferrin, complement C3 and CHI3L1 in the uterine fluid, in association with elevated estrogen levels, prompted us to address if the expression of these genes is related to reproduction. However, gonadotropin treatment of mice reduced the uterine expression of these genes in a model of in vitro fertilization. Our findings identify lactoferrin, complement C3 and CHI3L1 as highly expressed proteins in hydrometra fluid in association with chronically elevated serum estradiol levels. (C) 2015 Elsevier Inc. All rights reserved.