Mammary gland development is a complex physiological process regulated by a delicate balance of hormonal signals, among which estrogens play a pivotal role. Estrogens, primarily estradiol, mediate their effects through binding to estrogen receptors (ERs), notably ERα (ESR1) and ERβ (ESR2). These receptors are transcription factors that, upon activation, regulate the expression of genes involved in cell proliferation, differentiation, and apoptosis. The differential roles of ERα and ERβ in mammary tissue are critical for normal development and are implicated in the pathogenesis of breast cancer. The objectives of this research are to evaluate the endogenous and exogenous activation of ERα and ERβ receptors in various cellular contexts. This involves a detailed examination of the transactivation capacities of ESR1 and ESR2 in different cell lines by assessing the responses of ERα and ERβ to estrogenic compounds. Furthermore, the study characterizes the estrogen-induced responses in Esr2 knock-out mice, backcrossed onto a BALB/c background. This analysis sheds light on the role of ERβ in mammary gland development and tumorigenesis, as well as its influence on metabolism, providing a comprehensive understanding of its systemic effects. Additionally, the research explores the control of the estrus cycle and uterine disease treatment in equids. It examines the systemic effects of intrauterine device (IUD) placement in mares and investigates the efficacy of copper-banded IUDs in promoting bacterial clearance and preventing uterine diseases. This aspect aims to enhance reproductive health management in equids, addressing the control of the estrus cycle and the treatment and prevention of uterine conditions. Overall, these objectives are designed to provide a holistic view of estrogen receptor functions in both mammary gland and reproductive health, contributing to advancements in human medicine and animal husbandry.