1. The normal gaseous content of the swimbladder of fresh-water fishes near the surface is approximately that of the atmosphere. The composition varies with the species, pressure, temperature, amounts and kinds of dissolved gases, and with the seasons of the year. 2. When fishes are placed in water containing little or no oxygen the oxygen in the swimbladder diminishes; indicating that the swimbladder may act as a reservoir on which the blood may draw for oxygen in times of need. A perch is enabled to go into water of low oxygen content without asphixiation. 3. The effect of increased pressure in the surrounding water is to increase both the percentage of oxygen and carbon dioxide in the swimbladder. 4. If carbon dioxide is increased in the medium in which perch are living, the volume of the fishes is changed and the fishes automatically rise in the water. This response would be of adaptive value, causing the fish to move out of deeper water containing larger amounts of carbon dioxide into the safer zones above. 5. The primary function of the swimbladder of most of the fresh-water fishes is hydrostatic. 6. Perch apparently possess no voluntary muscular control over the size of the swimbladder. 7. Under conditions where high oxygen percentages were found in the swimbladder, a higher tension of the gases existed than in the blood. This indicates an active secretion. 8. Under conditions where fishes are not changing their depth rapidly, the gases in the swimbladder are probably kept constant by simple diffusion of gases from the blood. 9. A "rete mirabile" partially surrounds the walls of the swimbladder and furnishes a rich supply of blood. It is apparently the means by which the gases are transferred from the blood to the swimbladder. 10. The mechanism by which gas is secreted into the swimbladder can apparently be explained on a chemical and physical basis. The writer's experiments show that the hydrogen ion concentration of the swimbladder gland is increased by external stimulation. This indicates the secretion of a substance by the gland which may aid in the secretion of gases into the swimbladder. The apparent secretion of oxygen is believed to be brought about by (a) an increased flow of blood because of the dilatation of the capillaries and (b) an increased tension of the oxygen due to the local dissociation of oxygen from oxy-hemoglobin. 11. The swimbladder is a mechanism which enables the fish to actively maintain its stability in the midst of changing external conditions.