The Frank-Starling relationship of the heart yields increased stroke volume with greater end-diastolic volume and this relationship is steeper following beta-adrenergic stimulation. The underlying basis for the Frank-Starling mechanism involves length dependent changes in both Ca²⁺ sensitivity of myofibrillar force and power output. In this study, we tested the hypothesis that protein kinase A (PKA)-induced phosphorylation of myofibrillar proteins would increase length dependence of myofibrillar power output, which would provide a myofibrillar basis to, in part, explain steeper Frank-Starling relations following -adrenergic stimulation. For these experiments, adult rat left ventricles were mechanically disrupted and permeabilized cardiac myocyte preparations were attached between a force transducer and position motor and length dependence of loaded shortening and power output were measured before and after treatment with PKA. PKA increased phosphorylation of myosin binding protein-C (MyBP-C) and cardiac troponin I (cTnI) as assessed by autoradiography. In terms of myocyte mechanics, PKA decreased Ca²⁺ sensitivity of force and increased loaded shortening and power output at all relative loads when the myocyte preparations were at long sarcomere length (~2.30 µm). PKA had less of an effect on loaded shortening and power output at short sarcomere length (~2.0 µm). These changes resulted in greater length dependence of myocyte power output after PKA treatment; peak normalized power output increased ~20% with length before PKA and ~40% after PKA. These results suggest that PKA-induced phosphorylation of myofibrillar proteins explains, in part, the steeper ventricular function curves (i.e., Frank-Starling relationship) following -adrenergic stimulation of the left ventricle.