Experiments on the newtTriturus cristatus carnifexhave shown that the spleen of specimens, either anaesthetized with chlorobutanol or in a normal condition, stores red blood cells when the animal is well oxygenated and releases these into circulation under conditions of hypoxia. The extreme limits of congestion and decongestion, commonly encountered in nature, can be obtained in the laboratory by exposing the animals to the air or by immersing them in still water. During the shift from a congested to decongested state, the spleen is reduced to less than one‐fifth of its weight, while the erythrocyte concentration and correlated parameters in the blood stream increase by more than 60%. Splenic regulation of the circulating erythrocyte concentration compensates not only for variations in oxygen supply linked to environment, it may also compensate for the cyclical variations in red cell production of this amphibian, whose erythropoietic tissue activity is intermittent and subject to seasonal variation