Three groups of slaughter-type horses, totaling 30 mares and 29 geldings, were used to determine density effects on displacement (distance moved during a stop), falls, injuries, and orientation using a single-deck, open-topped commercial semi-trailer. Each horse was assigned to one of two treatments high density (1.28 m(2)/horse with 14 horses) or low density (2.23 m(2)/horse with eight horses). Both treatments occurred sequentially on the same day (treatment order was alternated each trial), using the middle 2.44x7.32 m compartment of a large semi-tractor trailer. The horses were transported for two laps around a 7.28-km course, averaging 25 min and 36+/-89 s. Each lap consisted of two 60 degrees turns, four 90 degrees turns, two 120 degrees turns, one 180 degrees turn, six hard brakes, and six rapid accelerations, which were more severe than conditions usually experienced in commercial transport. Displacement, falls, and orientation were recorded for each horse using overhead video cameras. Average displacement between the two densities was not different (P=0.47). The proportion of horses that fell in the high-density treatment (40%) was greater (P=0.046) than the low-density treatment (17%). The proportion of horses injured was greater (P=0.006) in high density (64%) than low density (29%). However, there was not a significant difference in the average severity of injury for the high-density treatment (1.77) versus the low-density treatment (0.92), P=0.48. Overall, the horses did not show a preference (P=0.38) for facing toward (47.5%) or away (40.7%) from the direction of travel and orientation did not differ (P0.18) between the high and low-density treatments. High stocking density of horses during transport increases the incidence of falls and injuries, and made it more difficult to get up when a subject was floored.