Vascular smooth muscle (VSM) maturation is developmentally regulated and differs between vascular beds. The maturation and contribution of VSM function to tissue blood flow and blood pressure regulation during early gestation are unknown. The carotid artery (CA) contributes to fetal cerebral blood flow regulation and well-being; we studied CA VSM contractility, protein contents and phenotype beginning in the mid-third of ovine development. CA were collected from early (88-101d gestation) and late (138-150d; term=150d) fetal (n=14), newborn (6-8d, n=7) and adult (n=5) sheep to measure forces in endothelium-denuded rings with KCl, phenylephrine (PE) and angiotensin II (ANG II), changes in cellular proteins, including total and soluble protein, actin and myosin, myosin heavy chain isoforms (MHC), filamin and proliferating cell nuclear antigen (PCNA), and vascular remodeling. KCl and PE elicited age- and dose-dependent contraction responses (P<0.001) at all ages except early fetal, which were unresponsive. In contrast, ANG II elicited dose-responses only in adults, contractility increasing >5-fold greater than fetal or neonatal (P<0.001). Increased contractility paralleled age-dependent increases (P<0.01) in soluble protein, actin/myosin, filamin, adult smooth muscle MHC-2 (SM2) and medial wall thickness and reciprocal decreases (P<0.001) in nonmuscle MHC-B, PCNA and medial cellular density. VSM nonreceptor- and receptor-mediated contractions are absent or markedly attenuated in midgestation and increase age-dependently, paralleling the transition from synthetic to contractile VSM phenotype, and in the case of ANG II, paralleling the switch to the AT1 receptor. The mechanisms regulating VSM maturation and thus, blood pressure and tissue perfusion in early development remain to be determined.