The article by Hess et al. explores the role of estrogen in the male reproductive system, challenging the traditional view that estrogen is primarily a female hormone. The authors demonstrate that estrogen, particularly in low concentrations, plays a crucial role in regulating the reabsorption of luminal fluid in the head of the epididymis. This process is essential for concentrating sperm, which is vital for fertilization and genetic variation. The study uses estrogen receptor (ER) gene knockout mice (ERKO) to show that disruption of this function leads to infertility due to diluted sperm in the epididymis. Additionally, the authors use in vitro methods to confirm that ER-α is necessary for normal fluid reabsorption in efferent ductules. The findings raise concerns about the potential effects of environmental estrogen exposure on male reproduction and the reported declines in human sperm counts. The study highlights the importance of understanding the physiological functions of estrogen in the male reproductive system and the potential risks associated with estrogen exposure.The article by Hess et al. explores the role of estrogen in the male reproductive system, challenging the traditional view that estrogen is primarily a female hormone. The authors demonstrate that estrogen, particularly in low concentrations, plays a crucial role in regulating the reabsorption of luminal fluid in the head of the epididymis. This process is essential for concentrating sperm, which is vital for fertilization and genetic variation. The study uses estrogen receptor (ER) gene knockout mice (ERKO) to show that disruption of this function leads to infertility due to diluted sperm in the epididymis. Additionally, the authors use in vitro methods to confirm that ER-α is necessary for normal fluid reabsorption in efferent ductules. The findings raise concerns about the potential effects of environmental estrogen exposure on male reproduction and the reported declines in human sperm counts. The study highlights the importance of understanding the physiological functions of estrogen in the male reproductive system and the potential risks associated with estrogen exposure.