The targeted addition and removal of DNA methylation marks are major epigenetic regulatory mechanisms essential for normal development. Ten-eleven translocation (TET) enzymes (TET1/2/3) mediate methylcytosine (5mC) hydroxylation, which can facilitate DNA demethylation and altered gene expression. Functions for TET enzymes during development of complex organs including the heart have not been explored. Using zebrafish strains with targeted mutations in TET genes, we identified Tet2 and Tet3 as the major 5mC dioxygenases during zebrafish embryogenesis and observed specific defects in cardiogenesis in tet2/3-/- double mutant larvae. Morphological, molecular, and reporter strain analyses indicate defects in both epicardium migration and atrioventricular canal (AVC) development in tet2/3-/- double mutants. Co-culture experiments suggest that the epicardial defect may be indirectly caused by an altered myocardial program. To investigate the molecular mechanism, we compared transcript profiles of embryonic hearts isolated from wild type or double mutant embryos, and identified several candidate pathways that could impact epicardial and AVC development. Genome-wide MeDIP-seq and hMeDIP-seq experiments are ongoing to correlate DNA methylation and gene expression changes between mutant and wild types. This analysis should elucidate for the first time essential DNA epigenetic modifications that govern gene expression changes during cardiac development.