The formation of new blood vessels by angiogenesis to provide adequate blood supply is a key requirement for the growth of many tumors. While normal blood vessels expressed the COX-1 enzyme, new angiogenic endothelial cells expressed the inducible COX-2. We evaluated the role of COX inhibitors in the mouse corneal micropocket assay in which angiogenesis is driven by the addition of a Hydron pellet containing basic fibroblast growth factor (bFGF). Neovascular areas were measured with a slit lamp five days after pellet implantation into the corneal stroma. All animals containing implants with bFGF (90 ng) developed intensive areas of neovascularization, whereas the controls implanted with the Hydron pellet alone did not. Indomethacin (a nonselective COX-1/COX-2 inhibitor) and SC-236 (a COX-2-selective inhibitor) inhibited angiogenesis in a dose-dependent manner. Importantly, the indomethacin-treated mice developed severe gastrointestinal toxicity at the efficacious dose of 3 mg/kg/day. By contrast, gastrointestinal lesions were not observed, and platelet COX-1 activity was unaffected, at anti-angiogenic doses of SC-236 (1-6 mg/kg/day). Furthermore, a COX-1-selective inhibitor, SC-560, was ineffective at doses up to 10 mg/kg, a dose that completely blocked platelet COX-1 activity in these mice. SC-236 was also effective in reducing angiogenesis driven by bFGF, vascular endothelium growth factor (VEGF), or carrageenan in the matrigel rat model. Finally, in several tumor models, SC-236 consistently and effectively inhibited tumor growth and angiogenesis. This novel antiangiogenic activity of COX-2 inhibitors indicates their potential therapeutic utility in several types of cancer.