We studied 1) ultrastructural features of adult mesenchymal stem cells (MSCs); 2) the differentiation potential of MSCs in a coculture with adult rat cardiomyocytes; 3) the early homing features of MSCs injected in the ventricular wall of a beating heart. a) Among the various morphological features detected, the presence of many small pseudopodia around the entire periphery suggests the capacity of the MSCs for migration within the receiving tissue. b) MSCs cocultured with cardiomyocytes present only preliminary evidence of voltage-dependent calcium modulation. Such a calcium modulation results uncoupled with the development of nascent or adult myofibrils, thus showing a limited lineage specification and a low plasticity to differentiate in a full cardiomyocyte-like phenotype. c) We used the isolated rat heart model to study the marker expression of MSCs implanted in normal hearts and in the border-zone of infarcted myocardium. In a subgroup of infarcted hearts a protective protocol of postconditioning was applied after infarcting ischemia. When injected in the ventricular wall of normal hearts, MSCs migrate very early through the interstitial milieu and begin to show morphological changes. Yet, in infarcted hearts MSCs remain in the site of injection forming clusters of round-shaped cells. Both in normal and infarcted hearts, we showed that, besides the proliferating cell nuclear markers, some transplanted cells early express myoblastic maker GATA-4, and some of them show a VWF immunopositivity. Moreover, a few hours after injection connexin-43 is well evident between cardiomyocytes and injected cells. In postconditioned hearts most MSCs remained concentrated in clusters located near the site of injection, but some MSCs are also detectable spreading in the myocardium far of the site of injection. We suggest that the isolated beating heart is a good model to study early features of MSC homing without external interferences. Overall the results show that MSCs (i) has a limited plasticity in vitro; (ii) MSCs can migrate within the organ; (iii) start to change marker expression few hours after injection into a beating heart. Data also show that infarcted myocardium and cardioprotective maneuvers influence transplanted MSC morphology and mobility within the heart.