Human umbilical vein endothelial (HUVE) cells plated on plastic or gelatin-coated dishes grow as a "cobblestone" monolayer. By contrast, endothelial cells cultured on a complex matrix (e.g., Matrigel) form three-dimensional, capillary-like structures. In the current study, we verified the capillary phenotype of the latter structures and asked whether the morphological changes induced by extracellular matrix also affect human endothelial gene expression and function in vitro. Concentrations of cellular fibronectin, prostacyclin, and endothelin-1 were measured in the conditioned media by enzyme-linked immunosorbent and radioimmunoassays. Steady-state concentrations of HUVE mRNA were estimated by reverse transcription-polymerase chain reaction and quantified by Northern analyses to assess fibronectin and endothelin-1 gene expression. We found that the subjacent extracellular matrix affects the morphology, proliferation, and differentiation of HUVE cells in vitro. Cells cultured on gelatin were more mitotically active, expressed significantly less cellular fibronectin, made similar amounts of prostacyclin, and secreted significantly more endothelin-1 compared with the same cells grown on a Matrigel substrate.