A mounting block for in vitro perfusion of the cat cornea is described. Using this apparatus and the techniques of Araie, the permeability (Pac) of the normal cat corneal endothelium to carboxyfluorscein was determined to be 2.5 plusmn; 0.2×10-4cm/min. To assess the sensitivity of this technique in determining changes in Pacassociated with alterations in endothelial, morphology, three cats underwent 2 successive unilateral, central, 10 mm diameter circular areas of endothelial debridement 6 weeks apart. Six weeks following the second wounding all 3 animals underwent morphometric analysis and Pacdetermination. A trend toward an elevation in Pacwith extreme reductions in cell density was observed.