The possible mechanisms in regulation of the respiration rate of mitochondria from liver of hibernating ground squirrels have been investigated. The inhibition of respiration has been shown to be mainly due to the inhibition of electron transfer to the respiratory chain from flavoproteins to cytochrome c. Calcium ions evoke the gradual increase in the respiration rate of mitochondria from liver of hibernating ground squirrels which is abolished by adding albumin, ruthenium red and sovcaine. The lower content of free fatty acids and decreased rate of the oxidation of exogenic NADH in the external pathway indicate the decrease in the activity of phospholipase A2 in mitochondria from liver of hibernating ground squirrels. The decreased calcium capacity of mitochondria indicates the higher sensitivity to calcium ions. A conclusion concerning the leading role of phospholipase A2 in Ca-induced activation of respiration of mitochondria from liver of hibernating ground squirrels is made.