The purpose of this study was to examine the effect of hydrogen peroxide (H 2O 2) on angiogenesis in cultured endothelial cells. Endothelial cells obtained from bovine thoracic aorta (BAECs) were cultured between two layers of collagen type I to measure the tube formation which is a marker for angiogenesis. Addition of H 2O 2 (0.1–10 μM) to endothelial cells for various periods increased the rate of tube formation. The maximum stimulation of the tube formation was obtained when cells were exposed to 1 μM H 2C 2 for 30 min, and the enhancement of tube formation was blocked by catalase (10 U/ml). Both proliferation and migration of BAEC which are known to affect angiogenesis, were also stimulated by the addition of H 2O 2 (0.1 and 1 μM). Thus relatively low concentrations of H 2O 2 stimulated angiogenesis, proliferation and migration. Ets-1 is a member of the ets gene family of transcription factors, which binds to the ets binding motif in the cis-acting elements and regulates the expression of certain genes such as proteases including urokinase plasminogen activator (u-PA) and matrix metalloproteinase-1 (MMP-1). Interestingly, H 2O 2 increased the ets-1 mRNA level in BAECs compared with the basal level. The H 2O 2-stimulated angiogenesis was completely blocked by an ets-1 antisense oligonucleotide, but not by a mismatched oligonucleotide. These findings indicate that low concentrations of H 2O 2 stimulate angiogenesis in BAECs, and the stimulation mechanisms may partially involve the enhancement of proliferation and migration. Moreover, the H 2O 2-induced angiogenesis is likely to be mediated by the transcription factor ets-1.