The morphological changes in endothelial cells derived from bovine brain microvasculature, carotid artery, and aorta during growth on extracellular matrices were compared. All cells formed tubular structures on a basement membrane. Ultrastructural studies showed that the tubular structures had lumens surrounded by many endothelial cells. On type I collagen gel, brain microvascular endothelial cells still formed tubular structures, but the other two cell types formed confluent monolayers. However, when a second layer of collagen gel was laid over these cells, tubular structures developed within 2-3 days. Brain microvascular endothelial cells form tubular structures more readily than endothelial cells derived from large vessels on both basement membrane and type I collagen gel.