Estrogen, traditionally considered a female hormone, is also present in males, where it plays a critical role in reproductive function. This study demonstrates that estrogen regulates fluid reabsorption in the male reproductive system, particularly in the epididymis. In male mice lacking estrogen receptors (ERKO), fluid reabsorption is impaired, leading to diluted sperm entering the epididymis and causing infertility. This suggests that estrogen is essential for maintaining sperm concentration and fertility. Estrogen receptors (ER) are present in male reproductive tissues, including the efferent ductules, which transport sperm from the testis to the epididymis. These ductules reabsorb fluid, concentrating sperm as they move through the epididymis. In ERKO mice, this function is disrupted, leading to fluid accumulation and testicular atrophy. The study also shows that estrogen is necessary for normal development and function of the male reproductive tract. In vitro experiments using ERKO and wild-type mice confirmed that estrogen is required for fluid reabsorption in efferent ductules. Anti-estrogen treatment reduced this function, further supporting the role of estrogen in maintaining sperm concentration. The study also highlights the presence of both ER-α and ER-β in male reproductive tissues, suggesting that estrogen may have multiple roles in male physiology. The findings have implications for human reproductive health, as environmental estrogens may interfere with normal sperm production and fertility. The study underscores the importance of estrogen in male reproductive function and raises concerns about the potential impact of environmental estrogens on human sperm counts.Estrogen, traditionally considered a female hormone, is also present in males, where it plays a critical role in reproductive function. This study demonstrates that estrogen regulates fluid reabsorption in the male reproductive system, particularly in the epididymis. In male mice lacking estrogen receptors (ERKO), fluid reabsorption is impaired, leading to diluted sperm entering the epididymis and causing infertility. This suggests that estrogen is essential for maintaining sperm concentration and fertility. Estrogen receptors (ER) are present in male reproductive tissues, including the efferent ductules, which transport sperm from the testis to the epididymis. These ductules reabsorb fluid, concentrating sperm as they move through the epididymis. In ERKO mice, this function is disrupted, leading to fluid accumulation and testicular atrophy. The study also shows that estrogen is necessary for normal development and function of the male reproductive tract. In vitro experiments using ERKO and wild-type mice confirmed that estrogen is required for fluid reabsorption in efferent ductules. Anti-estrogen treatment reduced this function, further supporting the role of estrogen in maintaining sperm concentration. The study also highlights the presence of both ER-α and ER-β in male reproductive tissues, suggesting that estrogen may have multiple roles in male physiology. The findings have implications for human reproductive health, as environmental estrogens may interfere with normal sperm production and fertility. The study underscores the importance of estrogen in male reproductive function and raises concerns about the potential impact of environmental estrogens on human sperm counts.