Adult Zebrafish (Danio rerio) can regenerate the heart after partial surgical resection of the ventricle (1,2). Less is known about the regenerative capacity of the embryonic zebrafish heart. The small size and fecondity of zebrafish embryo, their development external to the mother and their optical clarity, provide advantages for experimental analysis. Interestingly, zebrafish embryo appears to continue to develop even in the absence of normal circulation, facilitating the analysis of animals with congenital defects or induced damages (3). The aim of this study is to develop a new model of cardiac injury in the embryonic zebrafish by using a laser microbeam (4,5). Cmlc(Cardiomyocyte light chain)2:GFP transgenic zebrafish at 3dpf (days post fertilization) were divided in 3 groups: control (no treatment), sham (laser injury of caudal fin), treated (laser injury of heart ventricle). Embryos were anaesthetised in tricaine 20μM and positioned under a fluorescence microscope (x400). Videos of the heart were captured and analysed using ImageJ to determine ejection fraction (EF). A video edge detection system (IonOptix) was used to determine apex motion ampitude (AMA) and heart rate (HR). Then, heart ventricles or caudal fins, displayed on a monitor, were injured by a laser beam delivered by a CryLaser FTSS 355-50 mounted on a Zeiss light microscope. Video and edge detection of the heart ventricle in the sham and treated groups were assessed at 2hours and 24hours post-laser. Results: Data are expressed as mean ± SEM. In the control group (n=24) at 3dpf and 4dpf, the EF was 23.5 ± 0.9 % and 23.2 ± 1.1 %; the HR, expressed in beats per minute (bpm), was 148.7 ± 4.7 bpm and 152.1 ± 5.3 bpm; the AMA, expressed in μm, was 15.6 ± 0.9 μm and 16.7 ± 1.4 μm. In sham (n=20), EF was 21.6 ± 1.1 % before laser experiment, 20.5 ± 0.8 % 2h post-laser and 24.4 ± 0.9% 24h after; in treated, EF (n=18) was 22.4 ± 0.9 % before laser injury, 13.7 ± 1.5 % 2h post-laser and 25.2 ± 1.7 % 24h after (ANOVA test p<0,001 sham vs treated group 2h post-laser). The HR was 142.1± 5.6 bpm, 157.7 ± 7.2 bpm and 151.7 ± 5.7 bpm in sham and 154.8 ± 6.6 bpm, 122.5 ± 39 bpm, 116.8 ± 13.2 bpm in treated, respectively before laser injury, 2h and 24h post-laser. The AMA, in the same time course, was 11.9 ± 2.5 μm, 12 ± 1.2 μm, 13.1 ± 1.2 μm in sham and 12.7 ± 1.8 μm, 7.1 ± 2.7 μm, 9.9 ± 3.3 μm in treated embryos. In this study, we have demonstrated a new way to create damage in the zebrafish embryonic heart. This model may permit new ways of examining cardiac injury and recovery in the zebrafish. Further experiments are ongoing to investigate molecular pathways implicated in this process.