The presence of fluid in the mare’s uterus after natural mating or insemination is a normal event that occurs in response to the breeding. This is the result of an inflammatory response that is needed to rid the uterus of any excess sperm or other contaminating agents. The influx of neutrophils (white blood cells) into the mare`s uterus after breeding results in the elimination of sperm, bacteria and other cellular debris and causes the release of prostaglandins, which stimulates uterine contractions. Typically the inflammatory response peaks 12 hours after breeding and by 24 to 48 hours the mares uterus is empty of fluid. However, some mares are more susceptible to prolonged retention of fluid and it may be retained for several days after breeding.