Freeze-thaw injuries of rabbit corneas were treated with mesodermal growth factor (MGF) in organ culture and the repair of the endothelium followed by light microscopic examination of endothelial whole mounts. MGF-treated corneas showed an increased number of mitotic figures. In addition, MGF accelerated the rate of cell hypertrophy, formation of spindle-shaped cells, and the rate of their migration into the killed zone so that Descemet's was covered more rapidly than was that of the controls. In MGF-treated corneas, the cells lost their fibroblast-like appearance and became endothelial-like in appearance sooner than did the untreated corneas. A short period of treatment (6 hr) was sufficient to stimulate the repair processes described above. The healing responses obtained in organ culture for control eyes approximately those described in the literature for freeze-thaw injuries to the rabbit endothelium in vivo. The data presented also show that it is possible to accelerate endothelial regeneration in wound healing.