Water permeability across intact cornea and conjunctiva was measured by a dye-dilution method in which a cell-impermeant, photostable dye (Texas red-dextran, 3 kDa; Molecular Probes) was used as an inert marker of water flux. Texas red-dextran (0.05 mg/mL) was dissolved in hypoosmolar (∼150 mOsM), isosmolar (∼310 mOsM), or hyperosmolar (∼580 mOsM) saline and infused into the microchamber positioned over the cornea or conjunctiva. Flow was stopped after solution exchange, and Texas red fluorescence was monitored continuously over 4 minutes. In some experiments, the solution was supplemented with 10% (wt/vol) 500-kDa dextran to increase viscosity 10-fold. Solute-free water movement across the corneal surface produced linear changes in Texas-red concentration and measured fluorescence. Water flux, J v (in cubic centimeters per second), was computed from the product of chamber volume (V c), and the rate of fluorescence change (after background subtraction), d(F/F 0)/dt. V c was determined to be 3.8 μL when in contact with the curved corneal surface and 5.4 μL when in contact with the flat conjunctiva. With the assumption that unstirred layer effects are negligible, the osmotic water permeability coefficient, P f tiss (in centimeters per second), is defined from the relation: J v = P f tiss Sv w (Φ1 − Φ2), where S is the tissue surface area assuming a smooth surface, v w is the partial molar volume of water (18 cm3/mol), and (Φ1 − Φ2) is the osmotic gradient, giving: P f tiss = V c [d(F/F 0)/dt]/[Sv w(Φ1 − Φ2)].