In the retina, angiogenesis is an important component of normal physiological events such as embryonic vascular development. It is also involved in pathological processes including diabetic retinopathies and age-related macular degeneration, and tumour growth such as choroidal melanoma. Fibroblast growth factor (FGF) 2 and vascular endothelial cell growth factor (VEGF) are the two major angiogenic factors in the retina. We investigated the mechanism of proliferation and the regulation of the mitogenic properties of FGF2 and VEGF in cultures of chorocapillary endothelial cells (CEC). FGF2 is a strong mitogen for CEC and induced a 2.5-fold increase in cell proliferation after 4 days in culture in the absence of serum. In contrast, VEGF is a poor mitogen for CEC. FGF2, but not VEGF induces a large activation of MEK1, ERK1/2 and P90 RSK during CEC proliferation. Pharmacological inhibition of Ras processing, and of MEK1 and ERK1/2 activation reduced only by 50% FGF2-induced cell proliferation, suggesting that there is another signalling pathway for CEC proliferation. Pharmacological inhibition of the PI 3-Kinase also inhibits by half FGF2-induced CEC proliferation. FGF2 stimulates the activation of the PI 3-K, P70 S6K and Akt. Inhibition of both ERK1/2 and PI 3-K activities suppressed FGF2-induced CEC proliferation, demonstrating that CEC proliferation requires both ERKs and PI 3-K pathways. These data on the molecular mechanism and signalling may have important implications for providing more selective methods for anti-angiogenic and anti-tumoural therapy.