Transforming growth factor-beta 1 (TGF-beta 1) has been variably associated with the regulation of cellular proliferation and extracellular matrix expansion after arterial injury. We tested these associations in vivo in the rat carotid injury model. At 0, 3, 7, 14 and 28 days following arterial balloon injury, regional expression of TGF-beta 1 mRNA was assessed using in situ hybridization and the results compared to measures of cellular proliferation and extracellular matrix expansion. Both the TGF-beta 1 concentration measured in culture media of explanted carotid arteries and the quantitative in situ hybridization signal for TGF-beta 1 arterial media and neointima were maximal at 14 days after balloon injury. However, medial cellular proliferation was maximal at 3 days whereas neointimal proliferation was maximal at 14 days and significantly greater than medial proliferation. Neointimal cell density declined significantly between 7 and 14 days, indicating the expansion of extracellular matrix; however, medial cell density was unchanged between 3 and 28 days after balloon injury. Thus, differences in the regional arterial wall relationships between the time course of cellular proliferation, extracellular matrix expansion and the level of TGF-beta 1 expression demonstrate in vivo variability in the response to TGF-beta 1.