Purpose.To compare corneal endothelial cell function by measuring corneal thickness during temperature reversal between corneas stored in two different storage media, Optisol GS and Chen Medium (CM).Methods.Twenty paired corneas from 10 human donors were randomly assigned for storage at 4°C in Optisol GS (10 corneas) or CM (10 corneas). The storage media were masked, and measurements were done in a masked fashion. After storage for 48 hours, corneal thickness was measured by ultrasonic pachymetry at 2-hour intervals for 12 hours, during which time the corneas were perfused with BSS (balanced salt solution) Plus at 37°C. Scanning electron microscopy of two pairs of corneas from two donors was performed to assess ultrastructural change after 12 hours of warming.Results.Corneal thickness decreased during the first 4 hours of the warming period and then increased during the 6-to 12-hour warming period. These changes in corneal thickness over time were similar for the two storage media (p= 0.212). Scanning electron microscopy showed greater amounts of endothelial cell disruption in Optisol GS–stored corneas than those stored in CM after 12 hours of warming and perfusion.Conclusions.The endothelial pump of corneas stored in CM appear to be as well-preserved as those stored in Optisol GS, although greater endothelial disruption may be present with Optisol GS by scanning electron microscopy. Further studies are required to compare the clinical effectiveness of these two media.