Changes in the surface morphology of regenerating rabbit, rat and frog corneal endothelial cells in vivo have been investigated by scanning electron microscopy. In adult tissue these cells do not normally divide unless given a stimulus, such as injury. Surfaces of quiescent rabbit and rat cells are devoid of microvilli but display globular projections and surface pits up to 300 nm in diameter. However, regenerating endothelia are characterized by the appearance of microvilli which attain their greatest length when the cells are rounded. At this stage, cells also possess filopodia and broad processes. In cytokinesis, the microvilli have shortened and blebs and ruffles appear for the first time. In contrast to rabbits and rats, frog endothelial cells of noninjured tissue are covered by microvilli and smaller surface pits of 60-70 nm diameters. During regeneration, these cells have reduced numbers of microvilli and extensive foldings of the membrane. Neither blebs nor filopodia occur during the mitotic cycle and ruffles are not detected until cytokinesis.