A comparative study is conducted for the effect of one-, three- and six-hour artificial deep (20-19 degrees C) hypothermia on the content of free amino acids in the blood serum, tissue, nuclei and mitochondria of the rat brain. It is found out that the content is the highest in the blood serum after a three-hour cooling. In the brain tissue the amount of amino acids lowers, especially under conditions of a six-hour hypothermia. In nuclei a three-hour effect of hypothermia decreases sharply the content of free amino acids and the six-hour one increases the amount of most of them. Under hypothermia the content of nearly all amino acids in the brain mitochondria is higher than in the intact animals.