1. 1. The tubulin of both brain and testes of the amphibian Bufo spinulosus, Leptodactylus ocellatus and Odontophrynus occidentalis and of the fish Salvelinus fontinalis were partially depolymerized when the animals were maintained on ice for 90 min. 2. 2. Recovery of the animals at room temperature restored the normal degree of polymerization in both organs. 3. 3. The cerebral tubulin of the saurian Phymatura palluma and of the mammalian Myotis chiloensis instead, were not depolymerized in these conditions. 4. 4. The results suggest that the depolymerization in vivo of tubulin by lowering body temperature at 0–2°C, is common to many, but not all species of vertebrates. 5. 5. The existence of a cold stabilizing factor in the brain of some species is suggested. A comparison is made between these results and those of other authors who observed microtubule disaggregation by cold with the electron microscope.