1. The respiratory function of the swim-bladders of Polypterus senegalus was investigated. Experiments were carried out in tap water with an oxygen tension of about 140 mm. Hg.
2. Both swim-bladders were cannulated through the body-walls of the unrestricted fish. Gas samples were analysed for their oxygen and carbon dioxide content before and after the fish visited the surface.
3. A sharp increase in oxygen and a decrease in carbon dioxide tension was always observed after inhalation. This proves that atmospheric air is actually inspired into the bladders.
4. After inspiration, the amount of oxygen in the bladders decreased rapidly. This shows that oxygen is taken up by the blood, even when the oxygen content of the water is normal.
5. Inspiration of air is preceded by expiration which, on the average, reduces the volume of the bladders to about 40%.
6. The uneven distribution of inhaled air in the right bladder is shown to be due to anatomical configuration.