E xpression of behavioral estrus, or heat, in mares can have a profound negative effect on training and performance. Mares begin to exhibit sporadic periods of estrus in February or March, with the onset of follicular development during the spring transition period. Estrus is normally expressed for 5 to 7 days out of the 21-day estrous cycle during the physiologic breeding period. Owners and trainers often report variations in performance and temperament of mares attributed to the estrous cycle.1 Clinical signs attributed to performance problems included attitude changes, tail swishing, difficulty in training, squealing, horsing, excessive urination, kicking, decrease in performance, and colic-like discomfort associated with ovulation.2,3 It is important to confirm that the behavioral problem is truly associated with estrus or the reproductive cycle of the mare before initiating potentially unnecessary therapy. Accurate records should be collected to determine if the behavior occurs during a specific phase of the estrous cycle. In addition, a thorough physical examination should be performed to rule out other potential medical problems. Numerous techniques to suppress or modify the estrous cycle or estrous behavior of the mare have been tried. Attempts have been made to alter the time of estrus to avoid an event or performance, reduce the duration of estrus by inducing an early ovulation, or completely block the expression of estrus. It has been suggested that treatments for altering an undesirable behavior or performance problem may not require complete suppression of estrus.2 Therapy has included hormone administration, herbal supplements, nonsteroidal antiinflammatory drugs, acupuncture, intrauterine devices, and surgery. The ideal treatment for estrus suppression in mares would be safe, effective, easy to administer, reversible, and cost-effective. The goal of this article is to review existing treat-