The increase in rat body temperature by 2-3 degrees as a result of overheating (45 degrees C, 22% humidity) over 90 and 120 min is accompanied by changes in the rate of labeled precursors incorporation into rat liver protein fractions. The incorporation of labeled amino acids into liver nuclear matrix proteins within the first 90 min of overheating is somewhat decreased, whereas 120 min thereafter it exceeds by 30% the corresponding values in control animals kept at room temperature. The polypeptide pattern of the nuclear matrix in hyperthermia is characterized by an increased relative content of polypeptide components around Mr 100, 55, 40 and 30 kDa against a decreased level of several polypeptides as compared to the control.