The number of cardiac progenitor cells is a significant determinant of heart size (Keegan, Meyer, and Yelon, 2004). Several factors, including Bmp2, Fgf8, Nodal, and Wnt 11, have been implicated in promoting the assignment of cardiac identity (reviewed in Brand, 2003). However, much less is known about the opposing factors that set limits for cardiac progenitor specification. We have therefore also focused our attention on zebrafish mutants with large hearts composed of too many cardiomyocytes. Notably, our recent analyses of these mutants have revealed two potent mechanisms for restricting the formation of cardiac progenitors. These studies indicate that generation of the proper number of progenitors involves interplay between inductive and repressive pathways.