We have developed a robust in vivo small-molecule screen that modulates heart size and cardiomyocyte generation in zebrafish.Three structurally related compounds(Cardionogen-1 to Cardionogen-3)identified from our screen enlarge the size of the developing heart via myocardial hyperplasia.Increased cardiomyocyte number in Cardionogen-treated embryos is due to expansion of cardiac progenitor cells.In zebrafish embryos and murine embryonic stem(ES)cells,Cardionogen treatment promotes cardiogenesis during and after gastrulation,whereas it inhibits heart formation before gastrulation.Cardionogen-induced effects can be antagonized by increasing Wnt/β-catenin signaling activity.We demonstrate that Cardionogen inhibits Wnt/β-catenin-dependent transcription in murine ES cells and zebrafish embryos.Cardionogen can rescue Wnt8-induced cardiomyocyte deficiency and heart specific phenotypes during development.Currently,we are in the process of testing whether Cardionogen compound family stimulates cardiomyocyte and heart regeneration.These findings demonstrate that in vivo small-molecule screens can reveal compounds with cardiomyogenic effects and identify underlying target pathways.