A study was made on the roles of the Hering-Breuer reflex, the pneumotaxic center, the apneustic center, and the medullary respiratory center in the regulation of rate and depth of respiration in response to changes in blood concentration of carbon dioxide and oxygen. Integrated phrenic nerve discharge, abdominal respiratory movement, and intra-abdominal and intra-tracheal pressures were recorded in decerebrate cats as they breathed air, 5 % and 7% CO 2 in O 2,10% O 2 and, 100% O 2. Vagotomy abolished the rate increase but not the depth increase in response to CO 2 breathing. Pneumotaxic center ablation reduced the depth increase but exaggerated the rate increase in response to CO 2 breathing. Thus, the Hering-Breuer reflex normally controls the respiratory rate and, in the absence of the pneumotaxic center, maintains a relatively constant depth. The pneumotaxic center normally controls the respiratory depth and, in the absence of Hering-Breuer reflex, maintains a constant rate. During apneustic breathing, following vagotomy and pneumotaxic center ablation, both inspiratory depth and duration vary with the inspired CO 2 concentration. The apneustic center thus provides maximum depth and prolonged duration so that rate can be regulated by the Hering-Breuer reflex and depth by the pneumotaxic center. Following medullary respiratory center isolation, gasping rate was reduced by 100% O 2 but unaffected by 7% CO 2.