Abstract 1. 1. The blood-aqueous humor barrier in the eyes of the fresh-water teleost Salvelinus namaycush was investigated. 2. 2. The bicarbonate level in the aqueous humor does not appear to be in simple equilibrium with that in the plasma. 3. 3. Administration of Diamox, a carbonic anhydrase inhibitor, resulted in a significant lowering of the aqueous bicarbonate level which was incompletely compensated for by an increase in the chloride concentration of the aqueous. 4. 4. Diamox has no significant effect on hematocrit, hemoglobin or oxygen-carrying capacity of lake trout blood. 5. 5. The role of carbonic anhydrase in aqueous humor formation is discussed.