To develop an anatomically correct mitral valve prosthesis, one needs to study the dynamics of the mitral valve apparatus in vivo. The authors used three-dimensional (3D) sonomicrometry and custom visualization software to develop a system to study the mitral valve. Sixteen ultrasonic transducers each were implanted into the hearts of pigs under cardiopulmonary bypass. Four of these crystals were affixed to the base and apex of both papillary muscles, four were attached to the free edge of the anterior and posterior leaflets where the main chordae attach, six were placed around the mitral annulus, and two were affixed to the epicardial wall. The digital sonomicrometer system sequentially fired each transducer and listened for an ultrasound signal at the other 15. This process was repeated so that all 16 transducers were sequentially fired, and each cycle of 16 was repeated 200 times/sec. The matrix of distances obtained between all the combinations of pairs of the 16 transducers was converted to x, y, z coordinates, the shape of the mitral valve apparatus was reconstructed in 3D on a graphics computer, and the valve's motion was analyzed over several cardiac cycles. The authors conclude that mapping of the mitral valve apparatus in pigs by 3D sonomicrometry provides quantitative measurements that can aid in the design of a mitral valve prosthesis that closely replicates the anatomy and function of the natural valve.