Single- and split-dose irradiation of the eyes of dogs was performed with cyclotron-produced polyenergetic neutrons at a rate of about 0.2 Gy (20 rad) per minute. Neovascularization was subsequently induced surgically in the avascular corneas and stimulated for 7 days to study the response of slowly proliferating capillary endothelial cells. Corneal tissue samples were evaluated quantitatively by morphometric methods. A dose-response curve was drawn by plotting per cent capillary volume versus dose. D0 was 2 Gy (200 rad), and the recovery ability was determined to be equivalent to 1.3 Gy (130 rad). Depending on the level of damage, relative biological effectiveness (RBE) values were 1.9 to 2.3.