In the lungless salamander Desmognathus fuscus, mean body weight 4.5 g, the changes in total body concentration of adenosine triphosphate (ATP), creatine phosphate (CP) and lactate (LA) were measured during exposure to a severely hypoxic atmosphere (PO2 = 25 Torr) for 48 h at 13 degrees C. ATP and CP decreased, reaching a minimum at 3 h of exposure, and LA increased, attaining maximum values after 12 to 24 h of hypoxia. Thereafter recovery was observed and control values of ATP, CP and LA were reached after 48 h of sustained hypoxia. This behavior is attributed to a biochemical adjustment to hypoxia of the metabolic machinery which leads to normalization of chemical energy stores in spite of O2 uptake being persistently reduced to 30% of its normoxic level. The anaerobic energy yield derived from splitting of ATP and CP and from LA formation corresponded to about 2/3 of the oxidative energy deficit during the first 3 h of hypoxia. Thereafter anaerobic mechanisms were responsible for insignificant contributions to the energy balance.