Most horse owners administer oral deworming medication to their horses on a set schedule, often six times per year. The deworming process involves using a plastic syringe to inject a thick paste into the horse's mouth. Most horse owners do not specifically train their horses to accept this procedure. Consequently, many horses resist the procedure and some horses engage in behaviors, such as head shaking, pulling away, or even rearing, that may be dangerous to humans or to themselves. This study used a negative reinforcement shaping procedure to train six horses to accept dewormer medication. The procedure consisted of a food sampling phase followed by three shaping phases that simulated the deworming task, first using only the experimenter's hand, then a small syringe, and finally a large syringe. Once the horse was acclimated to the syringe, the horse's preferred liquid food was delivered through the syringe at the end of each trial. By the end of the study, all participants successfully completed the procedure and were able to stand still with no or minimal head movements while being dewormed.