Pairs of rabbit corneas were perfused and corneal thickness monitored. The epithelial side of the preparation, and after stabilization also the endothelial side, was covered with silicone oil. A constant thinning of the cornea was observed at a rate of 11·1 μm hr −1 (= 1·0 μm L h −1 cm −2) s.d. 2·1, n = 9. This thinning was also present after endothelial removal: 12·3 μm hr −1, s.d. 1·6 ( n = 5). Epithelial abrasion virtually abolished the thinning (2·5 μm hr −1, s.d. 1·89, n = 4), as did cooling from 34 to 0°C. When the preparation was perfused with a Cl-free solution (SO 4 2− instead of Cl −, corrected for osmolarity with sucrose) no significant thinning of the preparation was observed (2·4 μm hr −1, s.d. 2·49, n = 4) after covering both surfaces with silicone oil. This simple set of experiments quantified the epithelial pump mechanism. The epithelial pump rate of about 1·2 μm L hr −1 cm −2 has to be taken into account when endothelial pump rates are measured in the in vitro preparation with intact epithelium.