The combination of optical clarity and large scale of mutants makes the zebrafish vital for developmental biologists. However, there is no comprehensive reference of morphology and function for this animal. Since study of gene expression must be integrated with structure and function, we undertook a longitudinal study to define the cardiac morphology and physiology of the developing zebrafish. Our studies included 48‐hr, 5‐day, 2‐week, 4‐week, and 3‐month post‐fertilization zebrafish. We measured ventricular and body wet weights, and performed morphologic analysis on the heart with H&E and MF‐20 antibody sections. Ventricular and dorsal aortic pressures were measured with a servonull system. Ventricular and body weight increased geometrically with development, but at different rates. Ventricle‐to‐body ratio decreased from 0.11 at 48‐hr to 0.02 in adult. The heart is partitioned into sinus venosus, atrium, ventricle, and bulbus arteriosus as identified by the constriction between the segments at 48‐hr. Valves were formed at 5‐day post‐fertilization. Until maturity, the atrium showed extensive pectinate muscles, and the atrial wall increased to two to three cell layers. The ventricular wall and the compact layer increased to three to four cell layers, while the extent and complexity in trabeculation continued. Further thickening of the heart wall was mainly by increase in cell size. The bulbus arteriosus had similar characteristics to the myocardium in early stages, but lost the MF‐20 positive staining, and transitioned to smooth muscle layer. All pressures increased geometrically with development, and were linearly related to stage‐specific values for body weight (P < 0.05). These data define the parameters of normal cardiac morphology and ventricular function in the developing zebrafish. Anat Rec 260:148–157, 2000. © 2000 Wiley‐Liss, Inc.