Understanding the pathophysiology of equine back problems, for clinical evaluation, treatment or injury prevention, requires understanding of the normal 3-dimensional motion characteristics of the vertebral column. Recent studies have investigated regional vertebral kinematics; however, there are no reported measures of direct in vivo segmental vertebral kinematics in exercising horses. Relative movements between 2 adjacent vertebrae were recorded for 3 horses that were clinically sound and did not have a known history of a back problem. A transducer consisting of 2 fixtures and an array of liquid metal strain gauges (LMSGs) was used to measure 3-dimensional segmental vertebral motion. The transducer was attached directly to Steinmann pins implanted in the dorsal spinous processes of adjacent vertebrae in 3 vertebral regions: thoracic (T14 to T16), lumbar (L1 to L3) and lumbosacral (L6 to S2). Rotational displacements between adjacent vertebrae were calculated from the differential outputs of the LMSG array during walk, trot and canter on a treadmill. Peak magnitudes of dorsoventral flexion, lateral bending and axial rotation were recorded continuously for each stride. The largest motion of the 3 instrumented vertebral segments was at the lumbosacral junction. In general, the greatest magnitude of segmental vertebral motion occurred during the canter and the least during the trot. The dynamic and continuous measure of 3-dimensional in vivo segmental vertebral motion provides an important new perspective for evaluating vertebral motion and back problems in horses.