Female rats have been maintained at simulated altitudes 8,000-9,000 for 15-60 min. Under the hypoxic conditions there was noted an increase in the respiratory activity of fresh nonwashed liver mitochondria, using endogenous substrates in the uncoupled state, as well as coupling of the endogenous respiration with phosphorylation. After 40 min storage of the mitochondrial preparations from hypoxic animals in an ice-bath, an increase in the endogenous respiration and decrease in its coupling with phosphorylation occurred but the values did not reach the control level. Many parameters of respiration were increased in mitochondria hypoxic rats in presence of succinate: respiratory activity in the active metabolic state/V3/ and in the state V4, the respiratory control ratio, the rate of ADP phosphorylation, the coupling of respiration with phosphorylation in the state V4. All the changes found were considered as adaptive.