In this study, attempts were made to replace damaged myocardium of adult rats with embryonic grafts. To this purpose pieces of embryonic ventricular myocardium were prelabelled with 4',6-diamidino-2-phenylindole and placed into a damaged area of the host myocardium. The hearts containing the grafts were then examined between 2 days and 5-7 months later. Initially the 4'-6-diamindino-2-phenylindole labelled cells were localized only at the site of grafting, but by 2-5 weeks they migrated along the ventricular surface of the heart. Nevertheless the greatest density of grafted cells was always found in the damaged area. At all time points studied, the myogenic phenotype of the 4'-6-diamindino-2-phenylindole-labelled cells was maintained, as the cells contained myosin heavy chains. In addition, immunolabelling with antibodies against cardiac gap junction proteins revealed that initially gap junctions were scattered within the transplanted tissue but with time they became more organised, firstly by alignment into rows along the developing myofibres and then into structures that resembled intercalated discs. Thus the grafted embryonic cardiac myocytes survived in an adult host myocardium and expressed characteristics typical of heart cells.