The possible role of keratocytes in the neovascularization of the rat cornea was studied by means of light and electron microscopic autoradiography. The keratocytes were identified by labeling with 3H-thymidine. Subsequently, the neovascularization was induced by application of 5N sodium hydroxide to the cornea. Besides approximately 10% of the keratocytes, some of the wall cells of the new vessels also incorporated 3H-thymidine. Electron microscopic autoradiography showed that the labeled perivascular cell was extramural. Moreover, the cell seemed to secrete multilayer basement membranous material. Therefore, one might describe the labeled perivascular cell as an "adventitial cell" or "extramural pericyte". These data suggest that keratocytes may participate in the formation of neovascularization in the pathological corneal stroma of the rat.