Myocardial infarction results in irreversible loss of cardiac myocytes and heart failure. Tissue or cell grafting offers the prospect of reintroducing contractile elements into impaired hearts. However, implanted cardiac myocytes remain physically and electrically isolated from the viable myocardium. Accordingly, the proof of increased contractile function attributable specifically to cell grafting procedures is sparse. Over the last few years, we have developed a new method to generate three-dimensional engineered heart tissue (EHTs) in vitro from embryonic chick or neonatal rat cardiac myocytes. EHTs comprise functional and morphological properties of intact myocardium. We hypothesized that EHTs, preformed in vitro into suitable geometric forms, represent appropriate graft material for in vivo tissue repair with advantages over isolated cells. Herein we describe initial results from implantation experiments of EHTs in the peritoneum of Fisher 344 rats. EHTs survived for at least 14 days, maintained a network of differentiated cardiac myocytes, and were strongly vascularized. Thus, the present study provides the first evidence for the general feasibility of EHTs as material for a novel tissue replacement approach.