Cat corneal endothelium was damaged by a methylmethacrylate segment introduced into the anterior chamber. Within 3 to 6 months after each such operation the endothelial cell density (ECD) diminished, ranging from 1933 to as low as 450 (78% to 16%, respectively, of its preoperative value [PV]). Decrease in the ECD to between 1050 and 1150 (40%-45% of PV) made no impact on the corneal thickness (group 1: "non-swollen" corneas); further decrease was followed by an increase of the corneal thickness to a level of 11.5%-73.5% more than its preoperative value (group 2: "swollen" corneas). The coefficient of variation of ECD increased in both groups. The number of hexagonal endothelial cells diminished, while cells with three or nine or more facets appeared. All these changes were much more pronounced and verifiable in the group 2 corneas, which were distinguished by the appearance of giant endothelial cells, 2957-4095 micron2, ie, 7.5-13.5 times larger than the cells of undamaged endothelium. These data appear to indicate that the decrease of ECD to 1050 to 1150 (40% to 45% of its PV) does not provoke decompensation of the dehydrative function of the endothelium, whereas further decrease does provoke such decompensation, the giant endothelial cells probably being the main factor in this phenomenon.