The levels of NE and 5-HT were determined in different brain regions of the field rat and the guinea-pig under normal conditions and following cold exposure at 10 degrees C. Under normal conditions NE was found to be mainly accumulated in the midbrain while 5-HT exhibited its highest value in the pons plus medulla of both animals. Cold exposure of the field rat caused a general increase in NE concentrations while a general decrease occurred in the brain regions of the guinea-pig. 5-HT mostly decreased in the two animals. These changes in the amine levels could be related directly to the animal's response to cold.