InTriturus carnifex, the mechanism of respiratory compensation which regulates the quantity of circulating erythrocytes according to the environmental conditions is already active in larvae at Glücksohn's stage LXI. Larvae anesthetized with chlorbutol and kept at 27° C for 25 min show red blood cell (RBC) counts slightly below 70 000/mm3and hematocrit values around 17, while those kept at 6° C for 100 min ‐ the time necessary for the heart to beat the same number of times as at the former temperature ‐ show significantly lower values (P < 0.01): about 40 000 RBC/mm3and a hematocrit of 11. The percentage mass and histology of the spleens removed from specimens frozen in liquid nitrogen immediately after testing show that this organ is the site where excess erythrocytes are stored. Tests at 18° C show that, though the voluntary movements of the anesthetized larvae are completely blocked, their gills still move rhythmically, which prevents them from going into hypoxia in stagnant water as, instead, occurs in adults.