Use of high-speed treadmills for exercise testing and clinical evaluation of horses with poor performance has increased greatly in the past decade, writes Dr. Ray Geor in the October edition of The Horse. Although such exercise testing facilities are available only at some universities and larger equine hospitals, it is a common practice for veterinarians to refer horses with performance-related problems to one of these centers. The main advantage of treadmill tests is the ability to monitor closely the horse during exercise. More sophisticated measurements, such as oxygen consumption and gait analysis, can be made. In addition, it is possible to perform upper airway endoscopy while the horse is running, a procedure often crucial to the diagnosis of upper airway obstructions. It also is much easier to standardize the nature of the exercise test and the environmental conditions. What about disadvantages? One disadvantage already mentioned is that high-speed treadmills are expensive and not widely available. Second, the nature of the exercise undertaken often is very different from that performed on a track or similar setting. It can be difficult to mimic the same workload as that normally undertaken, in part because the horse exercises without a rider. In human athletics, this issue has been shown to be of great importance when evaluating performance--cyclists should cycle, runners should run, and rowers should row. Despite these limitations, the advent of clinical exercise testing on a high-speed treadmill has greatly improved our ability to diagnose conditions that contribute to poor exercise performance and, at least for now, is the gold standard for examination of horses during exercise. A common question asked by owners and trainers is, "Is it safe for my horse to run on a treadmill?" By and large, the answer is yes. Horses readily adapt to running on a treadmill. For most horses, only one or two training or acclimation sessions are required for them to appear comfortable running at different speeds on the treadmill. These training sessions can be performed on the day before the actual exercise test, but in many instances are done the same day. Another consideration for treadmill exercise tests is the tack worn by the horse. If upper airway endoscopy is to be performed, the horse should be outfitted with the head gear normally worn during performance.