Sixteen Quarter Horse-type geldings were used to determine the effectiveness of a pre-conditioning program on altering bone density in young horses prior to race training. The pre-conditioning program began when the average age of the horses was 15 mo. Horses were exercised on a high speed treadmill for 14 wk and were subsequently placed into race training. Changes in density of the third metacarpal bone were determined via radiographic densitometry throughout preconditioning and the race training regimen. Gain in total bone volume was greater in the group exercised during the pre-conditioning phase. Also, the pattern of bone density in different locations indicated that the exercised group might have been preferentially laying down more bone in the dorsal and medial cortices and less bone in the palmar cortex to achieve altered bone shape. Initiation of race training appeared to limit gains in bone density in comparison with the pre-conditioning phase.