Aerobic metabolic rates (MO2) and respiratory quotients (RQ = MCO2/MO2) were measured in hypoxic frogs, hibernating underwater for up to 90 days at 3 degrees C. After 3 months of submergence at a PO2 of 50 mmHg, MO2 levels were 25% of those seen prior to hibernation. In progressive hypoxia, a gradual shift in RQ from 0.85 to 1.0 signaled an increasing reliance upon carbohydrate metabolism. Moreover, the glycogen concentrations of skeletal muscle, liver and heart of hypoxic frogs were more rapidly depleted than in their normoxic counterparts. A plasma lactacidosis revealed that the hypoxic animals recruited anaerobiosis to fuel a large 'Pasteur effect'. Throughout all stages of hypoxia, cellular ATP levels were maintained homeostatic. The ability to depress metabolic rate such that ATP demands can be met by oxidative phosphorylation in an oxygen limited environment is the key to the frogs' overwintering survival.