It is generally accepted that gases such as CO₂ cross cell membranes by dissolving in the membrane lipid. No role for channels or pores in gas transport has ever been demonstrated. Here we ask whether expression of the water channel aquaporin-1 (AQP1) enhances the CO₂ permeability ofXenopus oocytes. We expressed AQP1 inXenopus oocytes by injecting AQP1 cRNA, and we assessed CO₂permeability by using microelectrodes to monitor the changes in intracellular pH (pH_i) produced by adding 1.5% CO₂/10 mM to (or removing it from) the extracellular solution. Oocytes normally have an undetectably low level of carbonic anhydrase (CA), which eliminates the CO₂ hydration reaction as a rate-limiting step. We found that expressing AQP1 (vs. injecting water) had no measurable effect on the rate of CO₂-induced pH_i changes in such low-CA oocytes: adding CO₂ caused pH_i to fall at a mean initial rate of 11.3 × 10⁻⁴ pH units/s in control oocytes and 13.3 × 10⁻⁴ pH units/s in oocytes expressing AQP1. When we injected oocytes with water, and a few days later with CA, the CO₂-induced pH_i changes in these water/CA oocytes were more than fourfold faster than in water-injected oocytes (acidification rate, 53 × 10⁻⁴ pH units/s). Ethoxzolamide (ETX; 10 μM), a membrane-permeant CA inhibitor, greatly slowed the pH_i changes (16.5 × 10⁻⁴ pH units/s). When we injected oocytes with AQP1 cRNA and then CA, the CO₂-induced pH_i changes in these AQP1/CA oocytes were ∼40% faster than in the water/CA oocytes (75 × 10⁻⁴ pH units/s), and ETX reduced the rates substantially (14.7 × 10⁻⁴ pH units/s). Thus, in the presence of CA, AQP1 expression significantly increases the CO₂ permeability of oocyte membranes. Possible explanations include1) AQP1 expression alters the lipid composition of the cell membrane, 2) AQP1 expression causes overexpression of a native gas channel, and/or 3) AQP1 acts as a channel through which CO₂ can permeate. Even if AQP1 should mediate a CO₂ flux, it would remain to be determined whether this CO₂movement is quantitatively important.