Observations on the rhythmic variation in the cat carotid body chemoreceptor activity which has the same period as respiration.

1. The activity in carotid body chemoreceptor afferent fibres in the cat has been recorded and found to have a rhythm with the same period as respiration.2. This rhythm is not an artifact; it is not due to arterial pressure changes with respiration nor to cyclical changes in pulmonary venous admixture. It is caused by changes in blood gas tensions during each respiratory cycle.3. The amplitude of the rhythm is modified by transient and long-term changes in inspired oxygen or CO(2) so that a rise or fall in O(2) or CO(2) tensions of arterial blood (P(a,O2), P(a,CO2)) from the physiological range reduces it. The ratio of the rhythm amplitude to the mean rate of chemoreceptor discharge increases with P(a,O2) over the range 40-240 mm Hg.4. The rhythm is modified by changes in respiratory frequency and volume.5. The fluctuations of arterial oxygen tension which have the same period as respiration are shown to be conducted up the vertebral artery at least as far as the vertebro-occipital anastomosis.6. It is proposed that the chemoreceptor rhythm reflects the moment to moment changes in blood gas tensions.