Myosins constitute a large family of molecular motors, hydrolyzing ATP and producing cellular movement. To date, a large number of novel isoforms have been found in muscle and non-muscle cells. Among non-muscle myosins, non-muscle myosin heavy chain (NMHC) II-A and II-B have been well characterized. An additional member of NMHC II-B, with a molecular weight of 220 kDa, was recently identified in bovine skeletal muscle. NMHC II-B proteins, in particular, have been suggested to be a useful early molecular marker for the detection of pathological conditions during acute or chronic organ rejection in which fibrotic changes occur. Since it is known that treatment with cyclosporine A (CsA), an immunosuppressive drug successfully used for preventing organ rejection and autoimmune diseases, is often associated with several side effects (hypertension and nephrotoxicity), the aims of this study were: (1) to demonstrate the homology of the new NMHC protein (220 kDa) in other mammalian species, such as Wistar rats; (2) to evaluate, by morphological and immunohistochemical studies, the possible changes induced by CsA treatment in NMHC protein (220 kDa) cellular localization and/or in its expression levels in myocardial tissue. First of all, our results showed a greater homology of the new NMHC within the same isoforms across species and between isoforms in the same specie; moreover, we observed that this protein increased following CsA treatment. This could be explained as a tentative of cardiac tissue to maintain the structural integrity of intercalated disks and so the contraction/relaxation process.