Two cell culture models were utilized to characterize the actions of peptide growth factors on the composition of the extracellular matrix of embryonic mesenchymal tissue. To model the three-dimensional architecture of mesenchymal tissue, chick embryonic mesenchymal cells were maintained in organ culture as adherent cell populations in small three-dimensional tissue spheroids and as sparse populations of cells embedded in a mesh of hydrated native collagen fibrils. Cell proliferation was stimulated by a variety of growth factors. All of the growth factors that elicited a mitogenic response in both of these culture systems also stimulated the deposition of an abundant fibronectin-containing extracellular matrix that colocalized with the regions of active cell proliferation. The suggestion that the matrigenic actions of growth factors for intact mesenchymal tissue are an integral part of mitogenic signaling is supported by the observation that surfaces derivatized with ProNectin, an artificial mimic of the RGD attachment domain of fibronectin, stimulated the proliferation of embryonic mesenchyme in the absence of exogenous growth factors. All of the growth factors that activated proliferation and fibronectin matrix accumulation stimulated the transformation of the mesenchymal cells into myofibroblasts that displayed the marker alpha-smooth muscle actin.