1. Gas secretion has been studied in three species of salmonid fishes, one of which accumulates both nitrogen and oxygen against considerable gradients, while the other two accumulate only nitrogen. None of these three species possesses a counter-current system (rete mirabile) which could step up a slight primary pressure effect.
2. Neither the nitrogen nor the oxygen capacity of the blood is large enough to account for the tensions found in the swimbladder by a simple one-step diffusion process.
3. One is forced therefore to consider a mechanism for active transport of these gases by the cells lining the swimbladder.