The respiratory role of the ventral surface of the medulla was studied in rats anaesthetized with a urethane-chloralose mixture. 2. In fifty-eight studies on twelve animals, direct superfusion of the medullary surface with artificial c.s.f. made acid by the reduction of bicarbonate content or by the increase of PCO2 produced no significant stimulation of respiration provided that the temperature of the brain surface was unaltered. 3. Superperfusion of the medullary surface with c.s.f. of low bicarbonate content produced an inhibition of respiration in fourteen of thirty-eight experiments. 4. Electrical stimulation on the surface revealed a localized area lateral to the pyramids and rostral to the XIIth nerve where stimulation at low intensity produced an increase in the frequency and depth of respiration. 5. The application of carbachol to a similar region increased both the frequency and amplitude of ventilation at lower concentrations than were required to obtain effects from surrounding areas. 6. Sudden switching between perfusates at different temperatures produced changes of ventilation within 1-2 sec of a change of surface temperature. The Q10 for the ventilation/temperature relationship was approximately 6. 7. The experiments confirm that the ventral surface of the medulla contains neural elements which, at least during urethane-chloralose anaesthesia, have a significant effect on respiration. The stimulus for these effects in the rat does not appear to be a change in H+ concentration. It appears more probable that the primary role of the area lies in the link between thermal and respiratory regulation.
