Hypothalamic temperature and the regulation of respiration of the ox exposed to severe heat.

1. Thermodes have been chronically implanted in the hypothalamus of the ox.2. During exposure of the ox to severe heat stress, rapid shallow panting changed to slower deeper panting when hypothalamic temperature (T(hy)) was approximately 40.5 degrees C. With subsequent rapid ambient cooling, hypothalamic temperature declined rapidly, and there was a reversion to rapid shallow panting before respiration slowly returned to normal.3. When hypothalamic temperature and rectal temperature (T(R)) had reached about 40 degrees C, raising hypothalamic temperature to about 41.4 degrees C did not cause the onset of slower deeper panting.4. When hypothalamic temperature and rectal temperature had exceeded about 41 degrees C and slower deeper panting was well established, lowering hypothalamic temperature to about 40 degrees C did not cause any reversion to rapid shallow panting.5. When hypothalamic temperature and rectal temperature had reached about 40 degrees C, holding hypothalamic temperature at 40 degrees C did not prevent the onset of the slower deeper phase of panting.6. Under moderate heat stress when hypothalamic temperature was approximately 39.9 degrees C and rectal temperature was approximately 40.4 degrees C, raising hypothalamic temperature at the same rate as that which was associated with the onset of slower deeper panting during severe heat stress, resulted in a parallel increase in the respiratory frequency and fall in rectal temperature; there was no change to the slower deeper form of panting.7. Holding hypothalamic temperature at 41.7-42 degrees C during post-heat cooling did not prevent the reversion to rapid shallow panting.8. It was concluded that the change from rapid shallow to slower deeper panting during severe heat stress, and the reversion to rapid shallow panting during subsequent cooling, were not the result of changes in temperature of the hypothalamic thermoregulatory centres.