1. Studies were made on the short-circuit current (s.c.c.) and fluid transport across rabbit corneal endothelium. 2. Normal s.c.c. of 27 muA.cm-2 is reduced to 19 muA.cm-2 in CO2-free Ringer, to 9 muA. cm-2 in HCO-3 -free Ringer and to zero in CO2 and HCO-3 -free Ringer. 3. Carbonic anhydrase inhibitors reduce s.c.c. from 27 to 19 muA.cm-2. Removal of exogenous CO2 causes no further reduction in s.c.c. 4. In CO2-free Ringer, net exogenous bicarbonate translocation is equal to s.c.c. 5. In all cases studied, net fluid transport across the endothelium in open circuit is directly proportional to s.c.c. 6. It is concluded that the endothelial 'pump' which regulates corneal hydration operates by 'pumping' bicarbonate ions into the aqueous humour. 7. Under physiological conditions, two thirds of the substrate is supplied by exogenous bicarbonate ions and one third is supplied by conversion of exogenous CO2 by intracellular carbonic anhydrase. 8. Metabolic CO2 does not participate significantly in the process, probably because it is at too low a concentration to compete effectively with exogenous CO2. 9. Electron histochemical studies suggest that carbonic anhydrase is located immediately underneath the posterior membrane of the endothelium, across which the active bicarbonate ion flux passes.