One‐day‐old mice hold a brief and robust capacity of heart regeneration after apex resection. We tested whether the rat displays comparable response and its impact in adult cardiac overall performance. Apical resection in rats at postnatal day 1 (P1) and 7 (P7) rendered comparable loss of cardiac tissue (16 vs.18%) estimated by magnetic resonance imaging. P1 was associated with robust cardiac regeneration at 21 and 60 days while in P7 rats there was essentially scar tissue replacement. Cardiac tissue perfusion in the neoformed region decreased in both P1 and P7 groups at 21 and 60 days using SPECT scanning. General cardiac function at 60 days was comparable among the groups, but under hemodynamic stress the degree of change on stroke work decreased significantly in P7 (sham: 18.4±4.0; P1: 47.0±17,8; P7: ‐0.5±7.5; P<0.05). Furthermore, P7 displayed increased chamber stiffness and interstitial collagen (P1: 6.9±0.6; P7: 11.4±0.6; P<0.05). Taken together, we provide evidence that adult P1 rats undergo robust neoformation of cardiomyocytes enabling preservation of overall cardiac function under hemodynamic stress despite decreased tissue perfusion. In contrast, P7 rats display mainly scar tissue replacement and compromised overall cardiac function.Grant Funding Source: FAPESP/Capes