Early in development, endothelial cells proliferate, coalesce, and sprout to form a primitive plexus of undifferentiated microvessels. Subsequently,this plexus remodels into a hierarchical network of different-sized vessels. Although the processes of proliferation and sprouting are well studied and are dependent on the angiogenic growth factor VEGF, the factors involved in subsequent vessel remodeling are poorly understood. Here, we show that angiopoietin 1 can induce circumferential vessel enlargement, specifically on the venous side of the circulation. This action is due to the ability of angiopoietin 1 to promote endothelial cell proliferation in the absence of angiogenic sprouting; vessel growth without sprouting has not been ascribed to other vascular growth factors, nor has specificity for a particular segment of the vasculature. Moreover, angiopoietin 1 potently mediates widespread vessel enlargement only during a brief postnatal period, in particular, prior to the fourth postnatal week, corresponding to stages in which VEGF inhibition causes widespread vessel regression. These findings show that angiopoietin 1 has a potentially unique role among the vascular growth factors by acting to enlarge blood vessels without inducing sprouting, and also define a critical window of vascular plasticity in neonatal development. Finding the key molecular factors that regulate this plasticity may prove crucial to the further development of pro- and anti-angiogenic therapies.