We then performed a chemical screen and identified several small molecules that increase or reduce cardiomyocyte proliferation during heart development. These compounds act via Hedgehog, Insulin-like growth factor or Transforming growth factor β signaling pathways. Direct examination of heart regeneration after mechanical or genetic ablation injuries indicated that these pathways are activated in regenerating cardiomyocytes and that they can be pharmacologically manipulated to inhibit or enhance cardiomyocyte proliferation during adult heart regeneration.