Three rabbit corneas each were stored in McCarey-Kaufman (M-K) medium, rabbit serum, and in a moist chamber at 4 degrees C refrigeration for various lengths of observation. The endothelial cells appeared normal under all conditions for the first 24 hours as compared with control corneas processed concurrently with each experimental group. After 48 hours of storage the specimens in the moist chamber showed isolated endothelial cell damage. The endothelia in M-K medium or rabbit serum appeared viable up to six days without significant differences although those stored in rabbit serum showed a better preservation of microvilli on individual endothelial cells. Under all conditions a mild shrinkage of the cells seemed to have taken place as indicated by the more pronounced cell boundaries. We incubated an equal number of control rabbit corneas at 37 degrees C with 5% CO2 and moist air in M-K medium, serum, and minimal essential medium (MEM) with 10% fetal calf serum and 100 units/ml of a penicillin and streptomycin mixture. In serum, the endothelia showed rapid destruction with swelling of the entire cornea. Those stored in M-K medium maintained a normal endothelial covering of the cornea up to six days. At nine days of storage, marked cellular changes were observed with dehiscence of the cellular layer. When stored in the MEM mixture, the endothelial cells showed a normal layer without obvious cell damage when compared with those stored in M-K medium up to four days. However, after six and nine days of storage, cellular destruction was greater in these specimens than in those stored in M-K medium. In addition, there was considerable swelling of the whole cornea under this storage condition.