Osmotically driven water flow across the corneal epithelium and endothelium has been measured as a function of temperature. For both membranes deviations from a single straight-line relationship are found in a logarithmic plot of hydraulic conductivity against 1/T. Both membranes show a high (14 to 16 kcal per mole) apparent activation energy at temperatures between 23 degrees C. and 5 degrees C. At higher temperatures, between 23 degrees C. and 37 degrees C., the apparent activation energy falls to 5.7 and 9.3 kcal per mole for the epithelium and endothelium, respectively. The low value for the apparent activation energy for water crossing the epithelium and endothelium at normal corneal temperatures presumably reflects water passage through water-filled channels.