twenty thoroughbred racehorses were ultrasonographically evaluated to determine the relation between normal values of the cross-sectional area (csa) of the right and left forelimbs superficial digital flexor tendons (sdft) in the metacarpal region for trained and untrained thoroughbreds racehorses. ultrasonography revealed that csa at 26cm distal to the accessory carpal bone is larger than other proximal levels, for either left or right forelimbs. in addition, the csa at 2, 4, 6, 8, 10, 12 and 14cm distal to the accessory carpal bone of the left forelimb are larger (p<0.05) for trained horses when compared with untrained horses. on the other hand, there was no significant difference (p>0.05) between left and right forelimb for csa of the sdft, at any level, for either trained or untrained horses, and there was no significant difference between untrained or trained horses for the csa of right forelimb. in conclusion, the csa of the left forelimb sdft for horses that had been in continuous race training remains larger when they were trained anticlockwise, contrary to horses that had not been training for more than one year.