The degradation products of hyaluronan are known to stimulate endothelial-cell proliferation and to promote neovascularization associated with angiogenesis, whilst native high-molecular-weight hyaluronan is inhibitory to these processes. To investigate the cellular signalling pathways coupled to hyaluronan-induced responses in angiogenesis, we have analyzed early-response gene expression in vitro, in cultured bovine aortic endothelial cells. Angiogenic oligosaccharides of hyaluronan induced rapid transient up-regulation of the immediate early genes c-fos, c-jun, jun-B, Krox-20 and Krox-24. In contrast, native hyaluronan when used alone failed to elicit a significant change in expression of any of the genes tested, and when used in combination with angiogenic oligosaccharides of hyaluronan, gave a dose-dependent inhibition of induced gene expression. However, prior addition of angiogenic hyaluronan, as little as one minute before addition of high-molecular-weight hyaluronan, abrogated this inhibition, suggesting that positive or negative responses associated with hyaluronan signalling are integrated at a very early stage following receptor binding. Conversely, prior addition of high-molecular-weight hyaluronan led to an irreversible block in gene expression and proliferative response. These data are consistent with native hyaluronan antagonizing the angiogenic response in part by blocking a signalling cascade at or immediately following ligand-receptor interaction. Finally, we demonstrated that chronic exposure to oligosaccharides of hyaluronan is essential for cell proliferation, indicating that short-term immediate early-gene signalling is insufficient to elicit the proliferation of endothelial cells.