The human endometrium undergoes regular cycles of synchronous tissue shedding (wounding) and repair that occur during menstruation before estrogen-dependent regeneration. Endometrial repair is normally both rapid and scarless. Androgens regulate cutaneous wound healing, but their role in endometrial repair is unknown. We used a murine model of simulated menses; mice were treated with a single dose of the nonaromatizable androgen dihydrotestosterone (DHT; 200 mg/mouse) to coincide with initiation of tissue breakdown. DHT altered the duration of vaginal bleeding and delayed restoration of the luminal epithelium. Analysis of uterine mRNAs 24 h after administration of DHT identified significant changes in metalloproteinases (Mmp3 and -9; P < 0.01), a snail family member (Snai3; P < 0.001), and osteopontin (Spp1; P < 0.001). Chromatin immunoprecipitation analysis identified putative androgen receptor (AR) binding sites in the proximal promoters of Mmp9, Snai3, and Spp1. Striking spatial and temporal changes in immunoexpression of matrix metalloproteinase (MMP) 3/9 and caspase 3 were detected after DHT treatment. These data represent a paradigm shift in our understanding of the role of androgens in endometrial repair and suggest that androgens may have direct impacts on endometrial tissue integrity. These studies provide evidence that the AR is a potential target for drug therapy to treat conditions associated with aberrant endometrial repair processes.-Cousins, F. L., Kirkwood, P. M., Murray, A. A., Collins, F., Gibson, D. A., Saunders, P. T. K. Androgens regulate scarless repair of the endometrial "wound" in a mouse model of menstruation. FASEB J. 30, 2802FASEB J. 30, -2811FASEB J. 30, (2016. www.fasebj.orgIn women, the most important tissues for biosynthesis of androgens are the ovaries, adrenals, and fat. In adult women, 50% of the testosterone measured in blood is secreted by the ovaries and adrenals, with a small midcycle peak (1). The other 50% is derived from conversion of adrenal and ovarian precursors, such as androstenedione, dehydroepiandrosterone, and dehydroepiandrosterone sulfate. Testosterone can be further metabolized to both estradiol and the potent nonaromatizable androgen dihydrotestosterone (DHT) in tissues that express aromatase and 5-a reductase enzymes, respectively (1-3).During the menstrual cycle, endometrial breakdown at the time of menses is the culmination of a tightly controlled cascade of biomolecular changes initiated by progesterone (P4) withdrawal (demise of the corpus luteum), which results in shedding of the upper functional layer of the endometrium. Detailed morphologic evaluation in women has revealed that breakdown and restoration of tissue homeostasis at the time of menstruation in normal women occurs rapidly, with epithelialization of the exposed basal stroma completed within 4 d of the first evidence of bleeding (4, 5). Piecemeal degradation and bleeding that give the surface of the endometrium the appearance of a wound has been documented with a hysteroscope intr...