This study examined the in vitro antiangiogenic effects of the somatostatin analog octreotide on the growth of human HUV-EC-C endothelial cells and vascular cells from explants of rat aorta cultured on fibronectin-coated dishes or included in fibrin gel. A total 10(-9) mol/L octreotide reduced the mean uptake of 3H-thymidine by HUV-EC-C cells by 37% compared with controls. The 10(-8) mol/L concentration of octreotide inhibited the proliferation of endothelial and smooth muscle cells growing on fibronectin by 32.6% and reduced the sprouting of cells from the adventitia of aortic rings in fibrin by 33.2% compared with controls, as measured by tetrazolium bioreduction and image analysis, respectively. These results demonstrate that octreotide is an effective inhibitor of vascular cell proliferation in vitro.