In heart failure, which is characterised by exercise intolerance, shortness of breath and oedema, the heart muscle is unable to pump a sufficient blood supply around the body. Cardiac muscle regeneration might thus restore function to a failing heart but how can cardiomyocyte regeneration be achieved? A zebrafish model of cardiac injury developed by Kenneth Poss and colleagues (see p. 3421) could provide valuable clues. It is known that adult zebrafish can regenerate cardiac muscle after surgical removal of about 20% of the ventricle. To study heart regeneration after larger injuries, the researchers created transgenic zebrafish in which destruction of more than 60% of the ventricular myocardium can be genetically induced. This massive myocardial loss triggers exercise intolerance in the fish, they report, but is completely reversed within 30 days through de-differentiation and proliferation of surviving cardiomyocytes. This new model of heart injury can now be used to understand why heart regeneration occurs in zebrafish – information that might help efforts to reverse human heart failure.