OBJECTIVE: To develop a method that allows quantification of the 3 anatomic rotations in the digital joints of moving horses and measure these rotations when horses are walking in a straight line on a hard track. ANIMALS: 4 healthy French Trotter horses. PROCEDURE: Triads of ultrasonic kinematic markers were surgically linked to the 4 distal segments of the digits of the left forelimb of each horse. Three-dimensional (3-D) coordinates of these markers were recorded in horses walking in a straight line. The three angles of rotation of each digital joint were calculated by use of a joint coordinate system as well as the 3-D orientation of the hoof and third metacarpal bone. A calibration procedure was developed to convert data from measurements within a technical coordinate system to data in relation to an anatomically relevant coordinate system. RESULTS: Precision of the method was 0.5 degrees, and repeatability of the calibrations resulted in variations of 1.4 degrees. Extrasagittal movements of the proximal and distal interphalangeal joints were obvious during landing because the impact of the hoof was on the lateral side. Mean +/- SD extension of the proximal interphalangeal joint was 10.0 +/- 2.5 degrees. CONCLUSIONS AND CLINICAL RELEVANCE: This study provides a description of the technical background, error analysis, and procedures used to measure the 3-D rotations of the 4 distal segments of the forelimb in walking horses. As a major result substantial involvement of the proximal interphalangeal joint in the sagittal and extrasagittal planes should incline investigators and clinicians to consider the functional importance of this joint.