The substitution of troponin in bullfrog skeletal myofibrils by rabbit skeletal troponin, troponin T and troponin T1 was examined by the densitometry of the SDS gel-electrophoretic patterns. The troponin in the myofibrils was replaced by the added rabbit skeletal troponin and troponin T (tropomyosin binding component of troponin) to almost the same extent. On the other hand, troponin T1, the N-terminal side fragment of two chymotryptic fragments of troponin T replaced troponin in the myofibrils less effectively than either troponin or troponin T. Troponin T, also replaced troponin in the myofibrils from rabbit and chicken striated muscles, less effectively than troponin T. This suggested that, in addition to the N-terminal side troponin T1 region, the C-terminal side region (troponin T2) of rabbit skeletal troponin T was also involved in the exchange reaction of troponin in the bullfrog skeletal myofibrils.