In myocardium, protein kinase A (PKA) is known to phosphorylate troponin I (TnI) and myosin-binding protein-C (MyBP-C). Here, we used skinned myocardial preparations from nontransgenic (NTG) mouse hearts expressing 100% α-tropomyosin (α-Tm) to examine the effects of phosphorylated TnI and MyBP-C on Ca2+sensitivity of force and the rate constant of force redevelopment ( ktr). Experiments were also done using transgenic (TG) myocardium expressing ∼60% β-Tm to test the idea that the α-Tm isoform is required to observe the mechanical effects of PKA phosphorylation. Compared with NTG myocardium, TG myocardium exhibited greater Ca2+sensitivity of force and developed submaximal forces at faster rates. Treatment with PKA reduced Ca2+sensitivity of force in NTG and TG myocardium, had no effect on maximum ktrin either NTG or TG myocardium, and increased the rates of submaximal force development in both kinds of myocardium. These results show that PKA-mediated phosphorylation of myofibrillar proteins significantly alters the static and dynamic mechanical properties of myocardium, and these effects occur regardless of the type of Tm expressed.