This study addresses whether the vasculature grows in proportion to the myocardium as the rat heart develops. The volume of myocardium and coronary vessels were estimated from micro-CT images of the hearts injected with Microfil® contrast agent. Young (n=5) and adult (n=5) hearts were scanned, resulting in 3D images comprised of 20?m on-a-side cubic voxels. The myocardial muscle and vessel lumen volumes were measured for all vessels 40 to 320?m in diameter by an erosion and dilation method applied to the binary images in which the contrast in the vessels were assigned '1' and all non-opacified entities were assigned '0'. The average total muscle volume increases by 50%, 129.4 to 237.4mm3, from young to adult rats, while the luminal volume increases by 10%, 16.6 to 18.6mm3. The vessel volume is 12% of the total muscle volume in young and 8% in adults. For a given vessel volume, the muscle volume in the young is 82% of the muscle volume in adults. We conclude that as the heart matures, the myocardium grows more rapidly than the vasculature. This may result in greater angles of separation between vessel branches, and the increase in myocardial coronary volume. The ratio suggests either higher blood flow velocity or a lower metabolic rate in adults.