The effect of a mild cold stress on activity of the key enzyme of the serotonin biosynthesis: tryptophan hydroxylase (TPH) in the brain-stem and the pituitary-adrenal axis response were studied in male mice of 11 inbred strains. Considerable interstrain differences in the TPH activity and an increase of the plasma corticosteroids level were revealed in the response to cold as well as differences in resistance against cold. A negative correlation was found between the TPH activity fall and the range of body temperature in the strains of mice under study. The decrease of TPH activity in response to cold seems to be due to the brain serotonin participation in the central thermoregulation.