In dogs anaesthetised with pentobarbitone sodium, raising the body temperature from 37 degrees C to 42 degrees C increased the rate of respiration and pulmonary ventilation but decreased the tidal volume. A similar change though of a lower magnitude, was observed in these parameters after cold blocking the vagi. At 40 degrees C body temperature, however, the vagal block was not effective in reducing the rate. The significance of hyperthermic panting mechanism dominating the Hering-Breuer mechanism at 40 degrees C has been discussed. It has been postulated that the vagi carry temperature dependent afferents that have effect opposite to the Hering-Breuer reflex. At 42 degrees C when respiratory failure usually sets in the temperature regulating function of respiration is the last to be lost.