Corneal stroma-endothelial preparations from 2-kg albino rabbits were incubated in vitro in a bicarbonate-Ringer solution equilibrated with 5% CO<sub>2</sub>-9% O<sub>2</sub> over 4 h in half-chambers at 37°C. An average net fluid pump activity of 6.8 μl/h was measured in the 4th hour. If such preparations were initially equilibrated with bicarbonate-Ringer for 90 min and then challenged for 20 min with 15m M sodium lactate or NaCl, an osmotically induced increase in fluid flow occurred. After removal of the challenge and return to bicarbonate-Ringer for 2.5 h, the net fluid pump returned to baseline and then increased again (to an average of 9.7 μl/h) for the lactate-but not NaCl-challenged endothelia. Scanning electron microscopy showed that lactate-challenged (but not the control or NaCl-challenged) endothelia contained a moderate number of very large pleomorphic cells within an otherwise regular mosaic. It is proposed that the large cells develop as a result of cell fusion (coalescence).