Obscurin A RhoGEF signaling is required for normal cardiac development. Cmlc2-EGFP transgenic zebrafish embryos injected with obscurin A RhoGEF morpholinos (B–F) were compared to control-injected transgenic embryos (A) at 72 hpf. Morphant embryos demonstrated a spectrum of cardiac abnormalities ranging from reduced cardiac function with cardiac dilatation and mild pericardial edema (B–D) to mild (E) or severe (F) ventricular hypoplasia and pericardial edema (<). (G–J) Cardiac functional assessments were performed in morphant (black bars) and control embryos (white bars) at 48 and 72 hpf. In the more mildly affected embryos with mild ventricular hypoplasia as determined by comparable end diastolic volume measurements relative to controls (I), there were significant reductions in heart rate (G), ejection fraction (H) and stroke volume (J) at 48 and 72 hpf compared to controls (∗t-test; p < 0.05).