The toxicity of elevated levels of very low density lipoproteins (VLDL, d < 1.006 g/ml) was investigated using porcine aortic endothelial cells in vitro. VLDL isolated from normal rat serum and added at elevated levels was as toxic as VLDL isolated from streptozotocin-induced diabetic rat serum. Injury was detected by scanning electron microscopy in 4-day-old primary cultures of endothelial cells after a 1 2 exposure to diabetic rat serum. Bleb formation and contraction was seen first in isolated cells ( 1 2 h), followed by cells at the periphery of the monolayer (1 h) and finally in cells throughout the monolayer (4 h). By 10 h few cells remained attached to the dish. A similar sequence of events occurred in 1-day-old cultures after a 3-h lag period. Serum from sucrose-fed as well as aged rats was also found to be toxic to endothelial cells in vitro. Elevated levels of VLDL were responsible for the toxicities of these sera. Scanning electron microscopy of the aortas from diabetic and sucrose-fed rats revealed endothelial desquamation, platelet and leukocyte attachment, fibrin deposition and the presence of microthrombi. The common occurrence of both micro- and macrovascular disease in diabetic, sucrose-fed, and aged rats and the toxicity of their serum in vitro suggest that elevated levels of VLDL may initiate vascular disease in these models.