To better understand the molecular mechanisms of regeneration and explore the potential signalling pathways as therapeutic targets for heart attacks. After treatment with the MEK inhibitor AZD6244 upon cardiac injury, the core members in MAPK/ERK signalling-mek and erk-demonstrate elevated expression, and these proteins are deposited at the injury site in zebrafish. pERK is also induced in non-cardiomyocytes near the injury site. Furthermore, the induced expression of a dominant-negative form of MEK1 inhibits zebrafish cardiac regeneration, characterized by increased cardiac fibrosis (a hallmark of regenerative failure), reduced or delayed production of regenerative myocardium, and migration of FLI1(+) endothelial cells, without direct inhibition of cardiomyocyte proliferation. Appropriate activation of MAPK/ERK signalling is essential for zebrafish cardiac regeneration.