We performed anterior radial keratotomy on the right eyes of nine rhesus monkeys, making eight incisions of various depths from 0.36 to 0.5 mm with a No. 76-A Beaver blade. After three months, corneal buttons were obtained from the operated on eyes of seven monkeys by penetrating keratoplasty and two monkeys were killed, and both operated and unoperated on eyes were enucleated. We examined the specimens by scanning and transmission electron microscopy. We saw linear protrusions on the posterior cornea beneath and parallel to the radial keratotomy incisions in all cases. There were swollen endothelial cells at the central cornea in three eyes. In eight corneas, we saw damaged endothelial cells with invading inflammatory cells. The control eyes showed no abnormal histologic findings. These findings suggested that deeper incisions cause more endothelial damage, possibly in relation to structural weakness produced in the cornea. We think that cuts in Bowman's membrane and in the stromal tissue may cause corneal stretching, resulting in a continuing process of injury to the endothelial cell layer. There may be long-term, continuing damage to the corneal endothelium after anterior radial keratotomy.