The small GTPase RhoA and its downstream effectors, ROCK1 and ROCK2, regulate a number of cellular processes, including cell motility, proliferation, survival, and permeability. Pharmacological inhibitors of the Rho pathway reportedly block angiogenesis; however, the molecular details of this inhibition are largely unknown. We demonstrate that vascular endothelial growth factor-A (VEGF) rapidly induces RhoA activation in endothelial cells (ECs). Moreover, the pharmacological inhibition of ROCK1/2 using 10 microM Y-27632 (the IC(50) for this compound in ECs) strongly disrupts vasculogenesis in pluripotent embryonic stem cell cultures, VEGF-mediated regenerative angiogenesis in ex vivo retinal explants, and VEGF-mediated in vitro EC tube formation. Furthermore, using small interfering RNA knockdown and mouse heterozygote knockouts of ROCK1 and ROCK2, we provide data indicating that VEGF-driven angiogenesis is largely mediated through ROCK2. These data demonstrate that Rho/ROCK signaling is an important mediator in a number of angiogenic processes, including EC migration, survival, and cell permeability, and suggest that Rho/ROCK inhibition may prove useful for the treatment of angiogenesis-related disorders.