Hypertrophy and hyperplasia play key roles in the growth of heart muscle. Following formation of the zebrafish heart tube at one day post fertilisation (dpf), it is currently thought that cell addition at the poles of the heart, with little proliferation of cardiomyocytes, results in growth until 2 dpf. From 3 dpf, cardiomyocyte (CM) proliferation contributes to formation of trabeculae, which increases muscle mass and cardiac efficiency. However, the relative contribution of hypertrophy and hyperplasia at these critical stages is not well known. We have characterised how the normal heart grows during the first five days. To understand the contribution of hypertrophy and hyperplasia in myocardial growth, we have acquired confocal and 2-photon images of CM labelled live hearts and quantified nuclear numbers, ventricular myocardial thickness, volume and dimensions. Analysis of fish with manipulation of genes implicated in cardiac hypertrophy will be presented.