The proliferation of the host endothelial cells during healing after penetrating keratoplasty (PKP) was investigated by exchange PKP in two groups of adult rabbits. After the endothelia of group A rabbits were selectively damaged by intracameral injection of benzalkonium chloride (BAK), grafts were made and transplanted to group B, untreated normal rabbits (damaged graft group). Grafts from group B rabbit eyes had already been made and these subsequently were transplanted to group A rabbits (normal graft group). Corneas were excised 4, 7, 14 and 28 days postoperatively, labeled with 3H-thymidine and subjected to autoradiography. The results indicated that in corneas with grafts damaged with BAK (damaged graft group), normal host endothelial cells proliferated and migrated into the damaged graft 4 days after surgery. In the normal graft group, the endothelial cells of the graft did not migrate to the damaged host cornea. On day 28, a monolayered apparently normal endothelium was regenerated in the damaged graft group. These results suggest that the proliferation of host corneal endothelial cells is vital in the endothelial healing of the graft after PKP in rabbits.