Injection of activated macrophages into the posterior vitreous of the rabbit induced vigorous fibrovascular proliferation over the optic disk and medullary rays, as demonstrated by 3H-thymidine autoradiography. One week after injection, endothelial cells and pericytes of the capillaries near the inner surface of the optic disk and rays were labeled; fibroblast-like cells, which were also labeled, migrated and formed vitreous strands. By the second week after injection, the fibrovascular tissue proliferated most actively, and traction medullary ray detachment and peripapillary retinal fold formation were observed. The cellular proliferation was accompanied by inflammatory cell infiltration. Glial cells within the optic disk, as well as retinal pigment epithelial cells beneath the detached retina, were labeled by 3H-thymidine. These results demonstrate that the fibrovascular proliferation originates from the vessel complex of the optic disk and medullary rays in this experimental model of retinal detachment.