We investigated the molecular mechanism by which troponin (Tn) regulates the Frank-Starling mechanism of the heart. Quasi-complete reconstitution of thin filaments with rabbit fast skeletal Tn (sTn) attenuated length-dependent activation in skinned porcine left ventricular muscle, to a magnitude similar to that observed in rabbit fast skeletal muscle. The rate of force redevelopment increased upon sTn reconstitution at submaximal levels, coupled with an increase in Ca2+ sensitivity of force, suggesting the acceleration of cross-bridge formation and, accordingly, a reduction in the fraction of resting cross-bridges that can potentially produce additional active force. An increase in titin-based passive force, induced by manipulating the prehistory of stretch, enhanced length-dependent activation, in both control and sTn-reconstituted muscles. Furthermore, reconstitution of rabbit fast skeletal muscle with porcine left ventricular Tn enhanced length-dependent activation, accompanied by a decrease in Ca2+ sensitivity of force. These findings demonstrate that Tn plays an important role in the Frank-Starling mechanism of the heart via on-off switching of the thin filament state, in concert with titin-based regulation.