Over the years, many factors have been investigated in the search for the cause and effective treatment of infertility in mares. Recently it has become apparent that many cases of infertility can be attributed to a failure of the mare's natural ability to remove contaminants from her uterus after breeding. During natural breeding, a stallion may deposit 50 milliliters or more of semen into the mare's uterus. Along with sperm and other compo-nents of semen, bacteria are also deposited into the uterus. The sperm necessary for conception are carried into the oviduct shortly after insemination and reside there until the time for fertilization arrives. The remain-der of the ejaculate, including bacteria and excess sperm, are removed from the uterus by mechanical clearance mechanisms with the bulk of the fluid passing through the cervix and being expelled through the vagina. Uterine contractions are a principle component of these mechanisms, clearing excess fluid and debris from the uterus within hours of breeding. If a mare is unable to clear the fluid from her uterus, the bacteria associated with breeding begin to multiply rapidly. In addition, the mare will transfer white blood cells and other components of the immune system into the uterus, further adding to the fluid present in the lumen. Studies have shown that infertile mares often have fluid remaining in their uterus at 24 or 48 hours after breeding while fertile mares do not. In fact, fertile mares have essentially cleared any excess fluid within 6 or 8 hours after breeding. Attention in the last few years has focused on methods to aid in removal of fluid from the uterus of infertile mares.

Administration of oxytocin has proven beneficial in subfertile mares by stimulating uterine contractions, thus expelling the fluid and increasing pregnancy rates. The effect of oxytocin on uterine contractions can be assessed by measuring intrauterine pressure. Prior studies investigating intrauterine pressure have used various types of catheters to record pressure changes but all previous systems have been plagued by various problems. We have been using a system, developed by coworkers in Germany, that avoids many of these problems. Our system reduces the problem of mare intolerance to the intrauterine device by using a small transducer placed in the uterus, which is connected to a small transmitter that can be secured to the mare. The data is then transferred directly to a computer by telemetry. The mare is free to move about in a stall or small area and is much more tolerant of a prolonged procedure than when she is confined in stocks and connected to a measuring device by wires. Moreover, by sending the data directly to the computer by telemetry, the problem associated with subjective evaluation of tracings on paper is similarly avoided. We have recently completed work examining changes in uterine pressure in normal mares after treatment with oxytocin before, on the day of, and after ovulation. Preliminary analyses have shown that the response to oxytocin varies depending on the day of the cycle on which the mare is treated. A greater response is observed when oxytocin is given before ovulation than after ovulation.