60 implantes of the left atrium myocardium were studied on the 1st, 3rd and 6th days of in vivo culture according to F. M. Lazarenko method (outbred laboratory male rats of 200-250g. body weight were used both as donors and recipients). Reactive changes of the cultured cardiomyocytes were assessed by methods of light and electron microscopy. Polymorphic nature of the cardiomyocyte reactive changes with the preservation of its tissue pecific determination was established. Myocardium implant did not form myosymplastic elements, cellular characteristics of organization remained in its implanted pieces. Data concerning degenerating and surviving cardiomyocytes are presented as well as those on cellular forms, identification of which is difficult on the ultrastructural level.