1. Rabbit carotid bodies were pre-loaded with [(3)H]dopamine (DA) synthesized from [(3)H]tyrosine and then mounted in a vertical drop-type superfusion chamber which permitted simultaneous collection of released [(3)H]DA and recording of chemoreceptor discharge from the carotid sinus nerve.2. The time course of the spontaneous release of [(3)H]DA (superfusion with media equilibrated with 100% O(2)) in the presence of monoamine oxidase inhibitors exhibited two linear components, an initial steep phase followed after 3-4 hr by a later slower phase of release.3. When a 5 min low O(2) stimulus was delivered during the initial steep linear component of resting [(3)H]DA release, there was an abrupt increase in release, the magnitude of which was stimulus-dependent.4. The efflux of total radioactivity from the preparation declined exponentially with time; under resting conditions it was principally non-metabolized [(3)H]tyrosine. During stimulation, however the efflux increased, and 60-80% of the radioactivity could be attributed to [(3)H]DA.5. For a given low O(2) stimulus, the ratio of [(3)H]DA release during the stimulus period over that in the preceding control period remained approximately the same throughout a single experiment. Ratios for different low O(2) stimuli (50, 40, 30, 20, 10 and 0% O(2) in N(2)) yielded a parabolic relationship when plotted against stimulus intensity.6. Transection of the carotid sinus nerve or removal of the superior cervical ganglion 12-15 days prior to the experiment did not affect the release of [(3)H]DA at moderate stimulus intensities (superfusion with media equilibrated with 30% or 10% O(2) in N(2)) but both procedures significantly depressed release at the highest stimulus intensity (100% N(2)).7. Chemoreceptor discharge and [(3)H]DA release were simultaneously monitored in experiments using superfusion media free of monoamine oxidase inhibitors. In these experiments, the efflux of [(3)H]dihydroxyphenyl acetic acid (DOPAC) was also measured. The increase in peak chemosensory discharge was closely correlated with the increase in total release ([(3)H]DA + [(3)H]DOPAC) during stimulation with a series of low O(2) stimuli.8. Release of [(3)H]DA was almost completely abolished during superfusion with Ca(2+)-free, high Mg(2+) (2.1 mM) media, and the stimulus-related efflux of [(3)H]DOPAC was significantly reduced. However, chemoreceptor discharge was diminished by only 55%. These data are discussed with respect to their implications for DA as a chemosensory transmitter in rabbit carotid body.