1. In the cat (decerebrate or under chloralose anæsthesia) denervation of the carotid sinuses usually decreases the rate of breathing and the minute ventilation. The same result is obtained if the vagi are cut previously. It is concluded that the chemical composition of the blood stimulates the sinus nerve-endings in the resting animal to discharge tonic excitor impulses to the respiratory centre.

2. The response to CO2 is usually reduced by denervation of the sinuses in the intact or vagotomised animal. It is concluded that the altered condition of the blood resulting from the raised CO2 concentration stimulates respiration in part reflexly from the carotid sinuses.

3. Vagotomy produces variable effects on the resting ventilation and on the response to C02. In animals under chloralose with sinuses denervated vagotomy frequently causes an enhanced reaction to C02. It is suggested that C02 may act differently on the various sensory components of the vagus nerve.

4. As a rule the response of the intact animal to CO2 is greater than that of the same animal with sinuses denervated and vagi divided.