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The extracellular matrix (ECM) is known to influence the phenotype of vascular smooth muscle cells (SMC) via the mitogen-activated protein kinase (MAPK) signaling pathways, in particular the extracellular-signal regulated kinase (ERK) pathway. A better understanding of the effects of ECM on SMC phenotype is important to developing functional vascular tissue substitutes. In this study, we used osteopontin (OPN) secreted into the media as a marker of the synthetic VSMC phenotype, to determine the effect of bovine collagen (BC), collagen-fibrin (BCF), and fibrin (BF) on SMC phenotype. Rat aortic SMC were cultured on two dimensional (2D) gels and films, and in three-dimensional (3D) gel environments, constructed with these ECM proteins. Tissue culture plastic (TCP) served as the control. Samples were collected after 1, 3, and 7 days in culture. An enzyme immunometric assay was used to quantify the amount of OPN and phosphorylated ERK. The geometry of the ECM resulted in marked changes in the pattern of OPN production. Cells cultured in 2D environments had the highest levels of OPN at day 1, dropping on day 3, and leveling off on day 7. For 3D gels, OPN levels and pERK increased steadily between days 1 and 7. There was a clear correlation between OPN secretion and activation of ERK, suggesting that ECM-mediated control of ERK signaling could used to modulate functional changes in SMC in engineered tissues.