A cardiac injury study in zebrafish reveals the plasticity of heart cell lineages as shown by a Notch-dependent transdifferentiation of atrial to ventricular cardiomyocytes, regenerating a cell type that is damaged in human heart failure. Obstacles to the use of stem-cell therapy in heart failure patients include the difficulty of ensuring the differentiation of cardiac progenitor cells into functional ventricular cardiomyocytes, and the delivery and integration of differentiated cells into the patient's ventricular myocardium. Neil Chi and colleagues studied the ability of specific cardiac muscle cell types to differentiate into closely related but distinct cell types in embryonic zebrafish hearts. They find that differentiated atrial cardiomyocytes can transform into ventricular cardiomyocytes when the heart is injured and that the Notch signalling pathway induces the regeneration. This work identifies an endogenous population of cardiac cells as a potential source for cardiac ventricular regeneration. Despite current treatment regimens, heart failure remains the leading cause of morbidity and mortality in the developed world due to the limited capacity of adult mammalian ventricular cardiomyocytes to divide and replace ventricular myocardium lost from ischaemia-induced infarct1,2. Hence there is great interest to identify potential cellular sources and strategies to generate new ventricular myocardium3. Past studies have shown that fish and amphibians and early postnatal mammalian ventricular cardiomyocytes can proliferate to help regenerate injured ventricles4,5,6; however, recent studies have suggested that additional endogenous cellular sources may contribute to this overall ventricular regeneration3. Here we have developed, in the zebrafish (Danio rerio), a combination of fluorescent reporter transgenes, genetic fate-mapping strategies and a ventricle-specific genetic ablation system to discover that differentiated atrial cardiomyocytes can transdifferentiate into ventricular cardiomyocytes to contribute to zebrafish cardiac ventricular regeneration. Using in vivo time-lapse and confocal imaging, we monitored the dynamic cellular events during atrial-to-ventricular cardiomyocyte transdifferentiation to define intermediate cardiac reprogramming stages. We observed that Notch signalling becomes activated in the atrial endocardium following ventricular ablation, and discovered that inhibiting Notch signalling blocked the atrial-to-ventricular transdifferentiation and cardiac regeneration. Overall, these studies not only provide evidence for the plasticity of cardiac lineages during myocardial injury, but more importantly reveal an abundant new potential cardiac resident cellular source for cardiac ventricular regeneration.