1. The gas in the vesicles (airbladders) of the brown alga Ascophyllum nodosum is conditioned by the formation of oxygen during periods of photosynthesis. 2. The walls of the vesicles were found permeable for gases, water, salts and sugar. Seawater will, however, only enter the vesicle when the pressure becomes negative. 3. An exchange of gases takes place through the permeable walls. Differences in partial pressures of gases between both sides of the vesicle wall are leveled. 4. Under natural conditions the pressure of the gas and the oxygen content depend on the intensity of photosynthesis. This is, however, counteracted by other factors. a. loss of gases by diffusion when the partial pressure of the oxygen in the vesicles is higher than in the surrounding water. h. loss of gases due to a saturation deficit in deeper water, e.g. at high tides. The gases then disappear into the surrounding water. Dam ant (1936). c. loss of oxygen by respiration. The nitrogen then also leaves the vesicle. The formed carbon dioxide is highly soluble and thus disappears. When these factors prevail over photosynthesis, a negative pressure may develop and seawater will be sucked in through the walls. This was observed on some vesicles in the month of December. It can also be induced under laboratory conditions.