1. Injection of CO2-saturated saline in a distal direction into either a vertebral artery or an internal maxillary artery in pentobarbitone-anaesthetized cats produced abrupt changes in respiration. Vertebral-artery injections produced a transient inhibition of respiration, followed by a stimulation of it. Internal-maxillary-artery injections produced only the inhibition.

2. Injections during inspiration usually shortened that inspiration, reduced its volume and prolonged the following expiration. In the first 30% of an expiration they prolonged that expiration, but given in the next 50% they shortened it. In the last 20% of expiration internal-maxillary-artery injections again slightly prolonged the expiration.

3. Phenyl diguanide injected into either a vertebral or an internal maxillary artery also produced abrupt effects on respiration.

4. The effects of CO2-saturated saline were abolished by intravenous acetazolamide, suggesting that nociceptors may be affected by a change in local pH.

5. The effects may arise from the excitation of vascular nociceptors, and our observations may suggest a way of studying in animals the receptors responsible for headache.

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