Thermal cautery of the peripheral cornea in rats caused proliferation of the limbal vasculature and invasion of the cornea. Tritiated-thymidine was used to identify premitotic activity in a total of 53,192 limbal vascular cells in five categories, viz., arteriolar endothelial cells, venular endothelial cells, arteriolar perivascular cells, venular perivascular cells, and capillary cells. From normal values in the range of 0.29 to 1.37%, the 2 h labeling indices reached a maximum of 13 to 14% in both endothelial and perivascular cells of venules and capillaries. Of particular interest was the finding of 18% labeling in arteriolar perivascular cells, and 7% in arteriolar endothelial cells. The categories showed a staggered onset of DNA synthesis, ranging from 17 h postcautery for capillary cells to 36 h for arteriolar endothelium and both arteriolar and venular perivascular cells. The duration of increased DNA synthesis also varied. Endothelial cells of both arterioles and venules showed narrow labeling peaks (12 to 24 h), while the adjacent perivascular cells and cells of the small vessels labeled for some 60 to 70 h. These results suggest that more than one stimulus to angiogenesis may be involved, or that the various cell types respond differently to the same stimulus.