This thesis contains the results of morphologic and functional studies of the endothelium of 253 consecutive penetrating corneal transplants. After keratoplasty, cells were lost from the central grafts at a rate of over 20% per year for approximately 3 years, after which the loss was minimal. Donor corneas preserved in organ culture had significantly more cell loss noted 2 months after keratoplasty than those preserved in M-K medium. The permeability of the grafted endothelium to fluorescein was significantly reduced in many cases; transplants with larger cells were less permeable. Transplanted corneas from older donors had more central endothelial cell loss and thicker corneas 2 months after keratoplasty than did those from younger donors. The central corneal thickness was less in phakic transplants than in aphakic grafts for 1 year after keratoplasty. After episodes of endothelial rejection had cleared, the affected grafts had lower cell densities and thicker corneas. To explain the results of studies of postoperative cell loss and endothelial permeability, two hypotheses were proposed: First, most endothelial cell damage occurs at the time of keratoplasty, with more cells lost from the peripheral graft and recipient cornea near the wound than from the central graft. Cells are "lost" from the central graft as they gradually move toward peripheral areas with larger cells; this slow process of cellular realignment, during which endothelial function is essentially normal, lasts for approximately 3 years. Second, as the cells in the central graft enlarge, the permeability of the central endothelium to fluorescein tends to decrease in proportion to the decrease in intercellular space adjacent to the anterior chamber.