We developed an animal model that allows the early phases of iris neovascularization to be studied in detail. Three major retinal branch veins were occluded with the argon laser in five eyes of cynomolgus monkeys, after which the eyes were enucleated at various time intervals. We observed three phases of the neovascular process in the iris. The early phase was characterized by vessel dilation and intense uptake of tritiated thymidine in the vascular endothelial cells. In the intermediate phase, prominent new vessels, ectropion uveae, peripheral anterior synechiae, and elevated intraocular pressure developed. Also noted were a decrease in tritiated thymidine uptake of the endothelial cells, a remarkable increase in stromal cell tritiated thymidine activity, and the formation of a neovascular membrane in association with the anterior migration of stromal cells. The late phase was marked by a further reduction of tritiated thymidine uptake and regression of the neovascular membrane.