<h3>Background</h3> Recent findings show that the neonatal mice retain a cardiac regenerative capacity up to 7 days after birth. The development of robust models to evaluate cardiac regeneration in mammals provide the unique opportunity to uncover molecular and cellular pathways that can regenerate myocardium after ischemic injury, findings that could be translated to human heart attack patients. In this regard, this study aims the establishment of cardiac injury models induced by apical resection and by cryoinjury in neonatal mice. <h3>Methods</h3> 1 and 7 days-old mice (P1 and P7) of BALB/c strain were anesthetized and submitted to cardiac apical resection (AR-group, n=37) or cardiac cryoinjury (CRYO-group, n=35). The cardiac function by echocardiogram (ECHO) and electrical activity by electrocardiogram (ECG) were evaluated 30 and 60 days post-operative (post-OP). Histological analyzes were performed 2, 7, 21 and 60 days after injuries. <h3>Results</h3> In the AR group, P1 mice showed significant reduction of QT and corrected QT (QTc) intervals and an increase percentage of collagen fibers (7 and 60 days post-OP) and cardiac function was preserved, suggesting cardiac regeneration (SHAM-group). P7 animals showed significant reduction in ejection fraction (EF), an increase in the percentage of collagen fibers (21 days post-OP) and cardiac tissue with disorganized fibers and picnotic nuclei. In the CRYO group, P1 mice showed significant reduction of EF, however, the values are still considered in the range normal (EF> 50%). In addition, they showed significant increase in end-systolic volume (ESV), end-diastolic volume (EVD), PR interval, and in the percentage of collagen fibers (21 and 60 days post-OP) and significant reduction in heart rate (HR). P7 animals showed significant reduction in EF and fractional area change (FAC), an increase in ESV, EDF and in the percentage of collagen fibers (7, 21 and 60 days post-OP), and cardiac tissue containing fibers and nuclei disorganized, suggesting ventricular dilation and functional impairment. <h3>Conclusion</h3> Our results suggest that the neonatal mice P1 hearts retain cardiac regeneration capacity after apical resection. The size of the injury may have negatively influenced the regeneration of P1 animals‘ hearts of the CRYO-group. In contrast, the hearts of P7 mice from the AR and CRYO-groups did not regenerate. Supported by CENABIO, CNPq, FAPERJ, DECIT and INCT-REGENERA.