The autocrine role of basic fibroblast growth factor (bFGF) in angiogenesis was studied in the rat aortic ring-collagen gel model using serum-free culture conditions. Immunohistochemical staining of the rat aorta showed bFGF in the cytoplasm of endothelial and smooth muscle cells. Aortic rings mechanically injured during the dissection procedure released bFGF, which was demonstrated in the conditioned medium by slot and Western blot analysis. bFGF-containing aorta-conditioned medium and purified bFGF increased both the number and length of microvessels sprouting from the explants. This effect was particularly evident during the second week of culture, when the release of endogenous bFGF was minimal. Neutralizing anti-bFGF antibodies induced a 40% reduction of angiogenesis. Regression of microvessels, which regularly occurred toward the end of the second week, was prevented by purified bFGF. These data support the idea that bFGF released by vascular cells plays an important role in the autoregulation of angiogenesis after injury.