Preferred speed is the behavioral tendency of animals to utilize a relatively narrow set of speeds near the middle of a much broader range that they are capable of using within a particular gait. Possible explanations for this behavior include minimizing musculoskeletal stresses and maximizing energetic economy. If preferred speed is determined by energetic economy (cost of transport, C(T)), then shifts in preferred speed should produce shifts in C(T). To test this hypothesis, preferred speeds were measured in trotting horses on the level and on an incline. The preferred trotting speed decreased from 3.29+/-0.24 m s(-)(1) on the level to 3.05+/-0.30 m s(-)(1) (means +/- s.d., N=6) on an 11.8 % incline. The rate of oxygen consumption was measured as a function of trotting speed on a treadmill and was a curvilinear function of speed in all horses under both conditions (level and 10 % incline). This curvilinear relationship resulted in a C(T) that was a U-shaped function of speed. The speed at which C(T) was minimal (i.e. at which trotting was most energetically economical) was very near the preferred speed on the level and decreased on the incline, again to a speed near the preferred speed on the incline.