Aims: To find an alternative to heart transplantation we investigated the possible integration of engineered heart tissue (EHT) after implantation in rat hearts. Methods: EHT was prepared from neonatal rats cardiomyocytes in collagen 1, matrigel and serum-containing medium. This mixture was cast in a circular design and electrically stimulated (1Hz, 1ms, 0,08mA) for 2 weeks. For implantation EHT was fixed around the beating rat heart. 30 days after transplantation, hearts were excised and transferred to a Langendorff system equipped with a 256 channel epicardial electrical mapping system. Hearts were perfused with Tyrode solution pH 7.4 for 45 min followed by 20 min perfusion at pH 6.5 for partial uncoupling of gap junctions. To investigate vascularisation of EHT hearts were perfused with dextranblue. Subsequently, hearts were analyzed by immunohistochemistry. Results: Prior to transplantation EHT exhibited Troponin I positive cardiomyocytes with typical striated pattern. EHT ring contracted spontaneously and developed contractile force (1mN). Furthermore, preformed capillaries were detected. Mapping experiments revealed that EHT in-vivo was electrically integrated and synchronized with the native heart. There were no arrhythmogenic foci or arrhythmic activity in the EHT. Functional vascularisation became evident from vessel staining after intracoronary dextranblue injection. Histological analysis showed that vessels were growing from the native heart into the EHT. Moreover, in the implanted EHT we discovered troponin I positive and cross strained cardiomyocytes. Surprisingly, there was also a development of elastic fibres. Conclusion: Under in vivo conditions EHT integrated with the recipient heart and emerged as a well vascularized and electrically coupled tissue. Supported by BMBF/TRM Leipzig (promotional reference 0313909)