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Cesarian section (C-section) has been described as a potential predisposing factor to retained fetal membranes (RFM) in mares. RFM should be diagnosed quickly, and therapy started to increase the likelihood of a positive outcome (Canisso et. al. JEVS. 2013; 33:570-579). This study aimed to describe the use of umbilical vessel water infusion for the treatment of RFM after elective C-section in mares (License CEUA 019854/2021-51). Pregnant Criollo type-mares (n=9), at 315 days of gestation, were evaluated daily to check their readiness for birth. Mares were submitted to an elective C-section when they showed udder development, waxing of the teats, and the pH of mammary secretions reached ≤ 6.4. All mares had RFM, as defined by the placenta not being expelled within 3 hours after careful extraction of the foal by C-section. Mares were randomly assigned into two groups: "Dutch technique" (n=4): use of low-pressure infusion of 0.9% saline solution into the umbilical vessel (Meijer et. al. AAEP Proceedings. 2015; 61:478-484); or "Counterweight" (n=5): use of small weights (e.g. 500-1000mL bottles of saline) tied to the bottom of the exposed fetal membranes (Turner R. AAEP Proceedings. 2007; 53:305-319). Oxytocin was not used after C-section or during RFM treatment. Mares that showed full separation and expulsion of the fetal membranes within 40 min of treatment starting were classified as responsive to treatment. The index of mares' responses among groups was compared by Fisher's exact test. Continuous data wereanalyzed by a Two sample T-test, and results were expressed as mean ± SD and ranges. Significance was set at p<0.05. There were no differences between gestational length (333±10, 328-360d), foal weight (36±3, 30-40 Kg), the time taken to perform the C-section (124±16, 90-150 min), and the time of uterine manipulation during C-section (57 ± 8, 45-70 min) between groups of treatment. As result, 75% (n=3/4) of mares showed full separation and expulsion of fetal membranes within 40 min of umbilical vasculature infusion. None (0%, n=0/5) of the mares with "tie small counterweights" responded within 40 min (p<0,05). The average time to release the fetal membranes in the Counterweight group was 5h (335±158, 180-600min, n=5) while mares that responded to the Dutch technique took 0.5h (30±10, 20-40min, n=3). Mares that experienced the Dutch technique did not show severe signs of discomfort, pain, or additional secondary side effects (i.e., uterine hemorrhage, uterine prolapse, metritis or colic) after the procedure. In conclusion, umbilical vessel water infusion (Dutch technique) is an effective, safe, and practical method for the treatment of retained fetal membranes after C-section in mares. Acknowledgments. The funding agencies CAPES, FAPERGS and CNPq. [ABSTRACT FROM AUTHOR]