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Cardiovascular Research Institute and Department of Physiology, University of California, San Francisco, California 94143, U.S.A.

1. The effects of dopamine on DC potential changes recorded from the carotid sinus nerve were studied in vitro using carotid bodies and associated sinus nerves removed from anaesthetized cats and rabbits.

2. The carotid body was placed in one compartment of a superfusion chamber and was superfused with buffered salt solution. The carotid sinus nerve was led into an adjoining compartment, containing lightweight mineral oil, for recording DC potential changes with chlorided silver electrodes.

3. Dopamine injected into the superfusing solution in amounts of 50-250 µg caused hyperpolarization of the DC potential. This effect occurred repeatedly in each preparation, the maximum number of tests in any one preparation being twenty-seven.

4. In the same preparations, acetylcholine (50-100 µg) and NaCN (10-20 µg) caused depolarizations of the DC potential, effects that have been described by others.

5. The hyperpolarizing effects of dopamine were reduced or abolished by the -adrenergic antagonists, dihydroergotamine (10-100 µg/ml.) and phenoxybenzamine (5 µg/ml.) and the dopamine antagonist droperidol (10-25 µg/ml.).

6. The results indicate that the inhibitory effects of dopamine on the frequency of carotid body chemosensory discharges in cats and rabbits are due to hyperpolarization of chemoreceptor afferent nerve endings and not necessarily to local vascular effects in the carotid body.