Objective. Cell death remains a major limitation of skeletal myoblast (SM) transplantation but the patterns of cell survival and proliferation in heart and their potential modulation by thermic stresses like heat shock (HS) and cryo-preservation (Cryo) are still incompletely characterized. Methods. To track SMs in situ, we developed a dual-marker system based on the semiconservative expression of the foreign soluble protein, β-Galactosidase (β-Gal) and the constitutive expression of the Y chromosome in a myocardial infarction model. Control medium or Lewis male rat SMs (fresh or subjected to Cryo or HS) were injected in Lewis female rats. Results. There was a massive cell loss early after transplantation in the fresh group, which was only partially compensated for by a subsequent proliferation. Conversely, both Cryo and HS significantly improved early cell survival but blunted subsequent proliferation so that, at 15 days posttransplantation, the total number of engrafted donor-derived Y-positive cells did not differ significantly between the three groups. Most of them expressed a skeletal muscle phenotype. Conclusions. These data confirm the high death rate of in-scar transplanted myoblasts, demonstrate the ability of those that survive to proliferate and differentiate along the myogenic pathway but do not support the efficacy of either Cryo or HS for increasing the ultimate magnitude of myoblast engraftment. Copyright © 2005 by Lippincott Williams & Wilkins.