<p>Myocardial sleeves around caval and pulmonary veins have ectopic activity, which is the main cause of atrial arrhythmias. The morphological and electrophysiological properties of this myocardium differ from those of the atria, but its mechanical activity in large animals has not been studied. We compared the phosphorylation of sarcomere proteins and the functional characteristics of myosin from the left and right atria, superior vena cava, and pulmonary veins of the porcine heart. Myosin from myocardial sleeves contains more of the β-isoform of the heavy chain, <i>i.e.,</i> is characterized by slower kinetics of cross-bridge attachment/detachment than atrial myosin. In myocardial sleeves, phosphorylation of troponin T and I, and tropomyosin was lower than in the atria. The phosphorylation features of thin filament proteins may affect the mechanical characteristics of cardiomyocyte contraction, as well as the mechanisms, regulating myocardial contraction, such as the length-dependent regulation and the mechanoelectrical feedback.</p>