Increasing the environmental temperature to 35 degrees C produced hyperthermia in 11 female rabbits, 80-90 days old. At the onset of hyperthermia the heart rate increased, but there was no further change after about two hours (range 1-3 hours); it remained constant at an increased rate until the end of the experiment. Blood pressure fell at first, but it had increased again after 90 minutes, and underwent no further change during the experiment. The mechanism of these circulatory changes were discussed in relation to the neutral control of body temperature regulation.