Stroma-endothelium preparations from rabbit corneas were mounted between two chambers and incubated with identical media on either side which contained different bicarbonate levels and, in some experiments, organic (Good's) buffers. Active fluid flow across the preparations was measured by means of a capillary tube attached to the stroma-side chamber. With media containing 2 to 50 mM bicarbonate (pH 6.2 to 7.8 in equilibrium with 5% CO2-air at 37 degrees C), the fluid pump was constant for at least 3 hr at a rate of 5 microliter/hr cm2 and was not significantly affected by the bicarbonate level. Over the same range of pH and bicarbonate but supplemented with 50 mM organic buffer, fluid pump was 8 microliter/hr cm2 for all bicarbonate concentrations used. Using Ringer solutions supplemented with 50 mM buffer (pH 6.3 to 8.4) but without added bicarbonate and in equilibrium with air, fluid pump was observed at approximately 4 microliter/hr cm2 at pH 6.3 and increased to 8 microliter/hr cm2 at pH 7.8. In all cases, fluid pump persisted for at least 5 hr.