Veterinarians are frequently called upon to help horse owners control the estrus behavior (“heat”) in mares used as performance horses or as show animals. When some mares come in heat, the hormonal and behavioral shifts that takes place distracts from their ability to perform their sport. Before deciding to manipulate your mare’s hormones, you should consult with your veterinarian. Sometimes a mare’s history can be suggestive of a hormonal problem. For example, a mare with a Granulosa thecal cell tumor of one of her ovaries often displays stallion like behavior.