Tissue, cellular, and subcellular morphometry of the thoracic aorta in 1-, 5-and 11-day-old rats was used to quantify the cellular hypertrophy and proliferation of smooth muscle cells and the absolute increases in volume of elastic laminae and collagen during the early postnatal period. In the 1-to 5-day interval, total wall volume increased 2.9-fold, wall thickness and the number of smooth muscle cells doubled, mean cell volume increased by 40%, and the volumes of elastic laminae and collagen increased 3.7-and 3.3-fold, respectively. From 5 to 11 days, circumferential growth of the aortic wall, without further thickening, produced smaller and unequal growth increments of each of its component structures, with increases in collagen (3.2-fold) > elastic laminae (2.3-fold) > muscle cells (1.6-fold). In the overall growth of smooth muscle cells (4.6-fold) from 1 to 11 days, only the cytoplasmic volume fractions of glycogen aggregates and rough endoplasmic reticulum were altered significantly (−78%, +32%). Certain aspects of normal postnatal aortic growth paralleled the response of adult aorta to experimentally induced hypertension.Circ Res 47: 417−424, 1980