The mitotic activity of the corneal endothelium during wound-healing was studied in 8 eyes of 5 cynomolgus monkeys. By transcorneal freezing, the endothelium was damaged at the corneal center over an area with a diameter of 2.5 mm. At various intervals, 3H-thymidine was injected into the anterior chamber and its incorporation into the endothelial cells was studied by autoradiography. On flat preparations of the endothelium, the total number of cells labeled with 3H-thymidine was counted on all specimens. The labeling of the cells occurred exclusively within the zone of about 2 mm from the center of the wound. The maximum labeling occurred 2 days after freezing and it decreased abruptly after 4 days, when endothelial covering of the central denuded area had been completed. No labeled cells were encountered after 30 days. The percentage of labeled cells to all cells present in a given area was computed and its regional distribution studied. On the second day after freezing, the maximum percentage of 12-19% was found in the center of the wound in one cornea and about 2 mm from the center in another cornea. On the third day, the maximum percentage of only 3-7% was found in the center and it decreased toward the periphery. It was thought that the number of labeled cells was too small to cover the damaged area by cell multiplication. It is suggested that the endothelial defect in the monkey cornea is covered mainly by migration of the adjacent cells.