Background: Thickening of basement membrane is a characteristic change in chronic diabetic complica- tions. Hyperinsulinemia and elevated insulin-iike growth factor-(I(F-1) might be associated with acceler- ation of atherosclerosis and the development of mi- crovascular complications. Ive have now determinerl that insulin and IGF-1 enhanced the expression of base- ment membrane components sucb as collagen and fi- bronectin in engothelial cc.lls and fibroblasts. Methods '. Dose-respvnsive effect of insuhn (1-1,000 ng/ml) and IGF-1 (1-500ng/ml) on collagen, fi- bronectin. And DNA synthesis by cultures of bovine capi]larly endo(helial cells, bovine arterial endot.helial cel)s, and hurnan skin fibroblasts were studied. The measurement of collagen and nancollagen prot.ein syn- thesis in these r,ells were accomplished by iabeling with ['H] proline and counting radioactivity in collagenase- sensitive and insensitive protein fractions. Fibronectin synthesis was measured by ELISA. DNA synthesis was analyzed by [H] thymidine incorporation into the cells. Results .' In bovine capillary endothelial cells, insulin and IGF-1 increased callagen and noncollagen pmtein (NCP) production, the latter was more prominent than the former. Insulin and IGF-1 also stirnulated the pro duction of collagen and NCP by bovine arterial endoth- elial cells but in a lesser extent as compared tr> the hovine capillarr endothelial cells. However, insulin and IGF-1 did not affect the production of fibronectm by endothelial cells. Insulin and IGF-1 significant]y in-creased DNA synthesis by bovine capillary endothelial cells but not by bovine arteriai endothelial cells. In human skin fibioblasts, insuJin and IGF-1 stimulated collagen, NCP, md DNA synthesis in a greater extent as compared, to the bovirw e@pillary endothelial cefls. ConchW'ons '. Hyyerineulinemia in diabetes might have direct effects on bovine eapilla,rly endothelial cells rather than boviae arterial endothelial cells.